Leitz Loc: Solms

According to the Df manual, entering the focal length only enables the use of power zoom with certain flash units. Entering both maximum aperture and focal length, besides displaying the aperture, only enables color matrix metering and improves TTL flash metering. It would be a major pain to go into the menu each time you zoom, so with those flash units it would be best to use an AF lens. Otherwise, I see no point in entering the focal length.
And, if you're not entering the focal length, you only need to assign a number for each maximum aperture, not every focal length.
Personally, I've never been in that menu - just set the aperture and adjust the shutter speed to suit, spot and centre-weighted metering is accurate. Haven't tried a non-AI lens yet, though, only stop-down metering, AI & AIS.

Papa j Loc: Cary NC

Leitz Loc: Solms

I'm not sure how well I can express this, but for most scenes where depth of field is important, I will use the focus indicator to nail the near and far distances, then set the aperture and focus according to the depth of field scale on the lens. Since I use single point focusing, when I compose that point may not be in focus and the indicator will not be lit. When the aperture stops down for the exposure, that point will be in focus (does this make any sense?).

Blasthoff Loc: Life halved NY and IN

The aperture is important to get right. The focal length, not as much, it could effect flash settings, such as auto zoom with Nikon flash units, so listing the shorter focal length of a zoom is safer to avoid "falloff". There are some "matrix" metering and focus detection considerations also. I myself avoid all matrix metering and focus detection problems by:

1 - By using a "split image rangefinder" focus screen I don't even pay attention to focus detection most of the time. It isn't necessary but it's there.

2 - Use "center weighted" metering if you have that option. It was more then good enough on Nikon film cameras for 25+ years. As a matter of fact, these two options bring focusing and metering right back on par with '80's Nikon film cameras where your lenses originated.