They call it a dust slinger. When I put in my new axels, I used the old axel seal to do the same thing.
Correct, Superior shafts dont come with them, and I found it impossible to get the stock ones off so I improvised with the old stock inner sealsAs already stated they are just to keep the big pieces of junk out. You can move them a little closer if you wish with a large flat bladed screwdriver or something, but I wouldn't worry about it. Further down the tube at the diff end is a seal that prevents anything getting in/out of the diff.
I have Superior shafts and don't have them at all.