Even if you are really adept at hand holding your camera and longer lenses, a monopod is alway a handy and useful accessory to have available in many different situations.

Yes! The rule of thumb is 1/ over the focal length is the minimum hand holding shutter speed therefore, a 300mm focal length requires a shutter minimum setting of at least 1/300 sec. Easy enough- but suppose you want to use a smaller f/stop to increase the depth of field and you don't want to alter the ISO setting. If you have a monopod you may be able to reduce the shutter speed somewhat, to accommodate the aperture of your choice. Monopod can be of great assistance if you are planning with action- subject in motion. It can help stabilize the camera where very critical focus and holding the camera steady and in place in place is important. It's not a steady and efficient as a tripod but it will oftentimes suffice where tripod usage is impractical or prohibited. I sometimes use my monopod to hoist the camera way overhead and shoot over a crowd or some other obstruction- with a wide-angle lens.

I carry my monopod all the time and hardly a week goes by where I don't need it.

Like many other photographic support accessories, monopods are very simple-basically, a one-legged - telescopic metallic length of tubular metal or carbon fiber. The successful and effective is a matter of technique using your body positions to reinforce the support of what is basically a stick. I have attached a website address- for some reason, I could not establish a link. This is a great little tutorial with very instructive drawings that illustrate the proper use of a monopod. Look it up- it a great 5 minute read!
Also attached- A few excerpts.