An exposure meter, AKA light meter or Flash meter is a very important tool in serious electronic flash usage in multiple flash studio-like setups, provided you know how to use it effectively and accurately. I assume the camera will be set in MANUAL mode and all TTL, automatic and command setups are not in play. The meter will assist you in determining basic exposure, determine the evenness of coverage of light in certain situations, and importantly assist you in arriving at lighting ratios between man, fill and accents light source in multiple lighting setups.

You will note that I use the word "assist". That is because the meter is not a magic device that will determine all of the aforementioned aspects of flash usage in one "click" of the activation button.

Firstly, you need to understand the correct method of making incident light readings - basically, holding the meter at the subject position and aiming the receptor dome towards the camera. This should get you well into the ballpark of a correct reading for a single light source- for example, a single light on or near the camera. Let's call that, in a portrait setup, the FILL light. You can place your MAIN light off the camera/subject axis and make individual readings. You can establish a ratio between the two basic lights. You may want a 1:2 to 1:4 ratio for various moods in portraiture. You can add more accent, background and hair lights to your setup and make certain they are all in range and subservient to the basic MAIN/FILL setup.

Sound pretty straightforward but here are a few issues that usually occur. The readings are made and seem to make sense but their resulting images are not what is expected- oftentimes too flat and the desired ratio was not achieved. This is due to UNSEEN SECONDARY LIGHT. When all your units fire simultaneously, there is lots of light bouncing from the surrounding walls and ceilings. It is UNSEEN because the modelling lamps usually will not show this effect. This can easily be solved by reducing the power to the distance of the FILL light. I metion this because it is especially prevalent in home-studio situations where the room may be small and brightly painted.

Another important issue: The light dome on most meters is collected light from multiple directions. This can work well in many available light and flash usage situations, however, when precisely setting I up ratios it is better to restrict the light receptor to one straight in the direction. Some of the older model meters had an interchange FLAT disk that replaced the dome for that purpose. The modern meters have a retractable dome that gobos the dome for more individual reading of ratios between units and between natural or ambient light and flash fill ratios.
The retractable dome will enable your making readings from individual light and also verifying reading on the highlight and shadow sides of a subject's face.

I have 2 flash meters in service. My youngest one is 15 years old- a Sekonic with all the aforementioned features. My Back up meter is a 25-year old Minolta mode with interchangeable receptors and both are extremely accurate.

Here are a few more points to consider. If you purchase a good meter and master its usage, you do not need any of those full-featured command exposure control systems- your radio transmitters need only serve to synchronize all the units.

If your studio setup is permanent, once you arrive at the basic setup, by trial and error, you may not requr a meter- you will get to know the power settings and distance to vary the effects, however. the meter is extremely handy if you make aesthetic variations, employ feathering, and various diffusion and modification devices. It makes it easier to integrate additional lights into your system.

A meter can help you create a CALLIIBRATED lighting system for location work. If your system is mobile you can make notations as to your ideal setup at home and easily replicate it at other locations by making individual readings of the various lights to match what you have established at home.

Of course, you can also use the meter for natural light as well.

As for brands, Sekonic would be my current choice if I were to replace my meters.

Many have added their two cents to this conversation. I have four plus decades as a pro and I will tell you from experience that I would never create a studio or location portrait without a handheld light meter. Just as with all of my equipment, I carry backups of my meter. I totally endorse what E.L. Shapiro and Burkphoto have stated. All clear, concise advice without the UHH babble speak.

Videos can help you learn, and so can a good book, but my suggestion to speed up and enhance your learning process is to study under an experienced pro who can get you past the many, many small errors you don't even realize you are making. Join PPA, or your local PROFESSIONAL guild/chapter and learn.

Photography can be a great hobby or profession, but please don't attempt to instruct without the credentials of experience. Several people have made some casual statements here that indicate to me that they are speaking without first-hand knowledge or experience. I am GUESSING that some "handheld meter opponents" have never tried/used one for more than a demonstration. Everybody has ignorance, just in different areas. We can all learn. I just ask that unless you have true experience in this endeavour, just sit back and listen to those who do have experience.

A good exposure meter is a very useful device but it is not a magical instrument that always provides perfect results unless you know exactly how to use it. You can buy a Spectra Cine Meter, which is a full-featured exposure meter that can be applied to still photography as
well. These meters have incredible accuracy, sensitivity, built-in calculators for contrast and ratio control and more. Their technical excellence earned an Acadamy Award for lighting
technology. Problem is, if you probe for your reading in the wrong area or don't know how to interpret the readings you get, you might get better results with the old sunny f/16 trick!

Investing in a good meter is wise IF you are going to BASICALLY learn how to use it as per the instructions and then progress to the more advanced metering techniques.

The meter will quantify light. The old meters requr you to transfer the reading to a calculator dial which combines the foot-candle number with ISO setting and shows you all the operator/shutter speed combinations. The venerated Weston Master metes had more complex dials that indicated various latitude ranges and were more useful when working with the zone system. Modern meters do many of these calculations and more by means of their circuitry and provide direct readouts on an LCD screen. Simple enough, but if you want to expose for highlights or shadows. intentionally under or over expose, determining the scene contrast is within your camera's dynamic range, been daylight and flash effectively, "shoot to the right", and determine lighting ratios, you sill have to know what you are doing to achieve the result you want.

There is absolutely nothing wrong with or inferior about using your camera's internal meter system and entirely negating handheld meter usage. If, however, you want to be able to carefully analyze a scene, solve more complex or difficult lighting issues, replicate a particular studio-like lighting setup at various locations, a meter will serve you well.

Meter usage is not overly complex or time-consuming. Once you know and understand how your meter works a few fast readings will provide all the information you need.

I used to carry my meter as a backup, in the olden days, the alwas packed my meter in case my (mechanical) camera failed. Nowadays, it is not likely the metering system will fail independently of the camera. If the camera fails EVERYTHING will go down with it. Can't take a picture with a meter so I have to pack a spare camera. I also pack a spare meer.

A good exposure meter is a very useful device but it is not a magical instrument that always provides perfect results unless you know exactly how to use it. You can buy a Spectra Cine Meter, which is a full-featured exposure meter that can be applied to still photography as
well. These meters have incredible accuracy, sensitivity, built-in calculators for contrast and ratio control and more. Their technical excellence earned an Acadamy Award for lighting
technology. Problem is, if you probe for your reading in the wrong area or don't know how to interpret the readings you get, you might get better results with the old sunny f/16 trick!

Investing in a good meter is wise IF you are going to BASICALLY learn how to use it as per the instructions and then progress to the more advanced metering techniques.

The meter will quantify light. The old meters requr you to transfer the reading to a calculator dial which combines the foot-candle number with ISO setting and shows you all the operator/shutter speed combinations. The venerated Weston Master metes had more complex dials that indicated various latitude ranges and were more useful when working with the zone system. Modern meters do many of these calculations and more by means of their circuitry and provide direct readouts on an LCD screen. Simple enough, but if you want to expose for highlights or shadows. intentionally under or over expose, determining the scene contrast is within your camera's dynamic range, been daylight and flash effectively, "shoot to the right", and determine lighting ratios, you sill have to know what you are doing to achieve the result you want.

There is absolutely nothing wrong with or inferior about using your camera's internal meter system and entirely negating handheld meter usage. If, however, you want to be able to carefully analyze a scene, solve more complex or difficult lighting issues, replicate a particular studio-like lighting setup at various locations, a meter will serve you well.

Meter usage is not overly complex or time-consuming. Once you know and understand how your meter works a few fast readings will provide all the information you need.

I used to carry my meter as a backup, in the olden days, the alwas packed my meter in case my (mechanical) camera failed. Nowadays, it is not likely the metering system will fail independently of the camera. If the camera fails EVERYTHING will go down with it. Can't take a picture with a meter so I have to pack a spare camera. I also pack a spare meer.