The rear cover protects the rear element, without it the rear lens is more vulnerable to being scratched or bumped if you put it down without a rear cap. However the lens is still perfectly useable without it. It also has a tab at 6 o'clock position signals the max aperture to certain cameras such as the FA, it is not used by any current model.
I don't quite understand the pictures you posted, as both have the rear lens cover in place. The top is from the later series-E version (chrome ring) and has solid rear protectors similar to those found on AI and AIS lenses. Early series-E lenses (black ring) use cheaper construction, the rear protectors and max aperture indexing post are simply prongs of sheet metal folded down from the rear plate. Simple but does the job.