Longer lenses give you a narrower angle of view. For that reason, you can fill the shot with a smaller item, or an item farther away. They bring things closer than shorter focal lengths.
They are also less forgiving when it comes to blur due to camera shake. You will most likely need to use higher shutter speeds when using them hand-held.
As for the AV-1, I used to sell them (almost 30 years ago

).
The AE-1 features shutter-preferred auto-exposure - you choose the shutter speed, and the camera chooses the aperture. Some people don't prefer to work this way, so Canon offered the AV-1, which features aperture-preferred auto-exposure - you choose the aperture, and the camera chooses the shutter speed.
At the time the AV-1 came out, most of Canon's competitors were featuring aperture-preferred auto-exposure.
I cannot recall what (if any) manual exposure options were available with the AV-1.
Matt