Me and a married friend of mine did an experiment one day at a club on base in Germany.
For two hours at a club, he wore his wedding ring. He isn't what anyone would call "HOT", but a decent looking guy. He had 8 women come up and "talk" to him.
For two hours after that, he took the wedding ring off. No women came up to talk to him.
Makes you wonder what single women are looking for huh?
I'm just amazed anyone could spend four hours in a club. In all my thirty years on this planet, I've never even been in one, nor wanted to.
I have eaten club sandwiches though, so I know of which I speak.
That's just a demonstration of the concept of "social proof". The ring demonstrates to any woman watching that the man is likely to be stable, reliable, probably at least a respectable earner, and experienced at sex and relationships. And women are always attracted to men they perceive to be experienced leaders.
There are, of course, ways for single men to demonstrate social proof without wearing a fake ring, but they are beyond the scope of this topic. The popularity of your ring-wearing friend, though, is testimony to how few single men have any idea what they are. ;)
My theory is evolutionarily hardwired Bayesian analysis. If a man is a quality mate, he's more likely to be in a relationship. Thus in the absence of any other information, a man in a relationship is more likely to be of high quality. Women apply this reasoning unconsciously and find themselves more attracted to involved men.
I remember a Seinfeld episode about that. George Costanza finally started wearing a wedding ring just to have a chance with females. :)