What wax? Why do surfers have wax on their chests? To remove chest hair?

I'm determined to get to the bottom of this rash thing.

If rash is not a problem for catamaran sailors, why can't these things have some other name?

P.S. I just figured out that surfers put wax on their boards, but cat sailors don't put wax all over their decks, and most of us don't lie on our decks with bare chests, so why do we need rash guards?