These are in the mid 80s and I had that on my 50mm and this same 28mm lens ( among a few other AF lens ) back then but I don't recall them having any reasons or use like a possible attachement.
I think it's more a product design as it was the first AF lens from Nikon. It did not last as many complained it's too thin to be useful. heh. By the next update version they change to the rubber ribbed and twice as thick ring version.
If there ever was a use, it would be to put some kind of attachment ring on the original focus ring to make it easier to focus. But that's just me speculating.