There have been a few topics made within the last week regarding cock rings. Now, I don't have any experience with these, so I'm going by what I've seen what others have posted.
I think that cock rings give your penises harder erections since they're tight against the skin and blood becomes trapped in the engorged penis tissue. I also presume that men wear them while flaccid, as well, to give their penis a boost in the bulge department since it seems to push everything (cock and balls) forward instead of hanging down.