I've used one for 30 years...I think it's a Saunders brand. Plastic, and has a hole in it for the string to come through.
Probably the main reason I started using one was because of hunting out of mesh metal hang on stands. I would often have the bottom limb tip resting on the metal platform. I never wanted the tip to be in contact with the metal stand. It's never squeaked and, if anything, it cuts down on the chance of the limb tip bumping against the metal stand, step, or whatever.
I've never felt the need to put one on the top limb.
And I will often use the bow as a rest to steady myself when glassing. Bottom limb resting on the ground (or stand), and I grab the binoculars and upper limb with both hands. I'm much more steady and effective at glassing that way. Plus, if I have to make a quick move, I already have my hands on my bow...no need to reach for it on a hanger.
Try resting your binocs on your bow when glassing...much less eye fatigue doing it that way.