It's proof of capability. Sort of like peacock feathers. "Look, I can carry around this obscene tail and still not get eaten! How's them genetics?"

For humans, it's, "I have excess resources I can afford to burn according to the socially-accepted ritual or test, so when you bear my offspring, you can be sure I will also have excess capacity to provide for both you and your offspring."

A female may have second thoughts about choosing you as a mate if you appear to cheat at the test or don't do the test right.