They're there for keeping the limb snug in the pocket, mostly to reduce noise. Some of the early wood risers also had them. Later versions don't have them, I suppose because they're an extra expense and really not needed. I rather like them.
Over nearly 50 years, there certainly are small differences, inconsistencies from one set of limbs to another. The rubber bumpers helped account for those. Now I occasionally improve fit and quiet limb noise with moleskin, tape, small felt pads and/or small nylon cabinet stops.