Quoted from yancy:I always assumed those rubber rings were to prevent vibration damage to the lamps, but at the same time that seemed like a far-fetched level of care for a manufacturer. I guess they just didn't wanna stock shorter screws.
IMO, they also did that for ease of assembly. Probably had a team assembling screw/socket/rubber assemblies and putting them in a bin, then the linemen need not worry about steadying a screw as the whole socket assembly was bound together by the rubber.