There doesn't have to be an advantage.
That's the point of sexual selection. Think of a male peacock. Its tail is a disadvantage, to grow it takes resources away from the rest of the body, and makes it more difficult to escape from predators, but because peahens like a large colourful tail it evolved anyway.
( The Awful Puppy Bakery is still bored, Wed 12 May 2010, 19:29, Reply)