Why do tennis players have orgasms everytime they hit the ball?

The other answers are right with the technical stuff, however i do wonder if it's really that useful. I mean, Federer, the most successful player ever makes very very little noise. If he can hit the ball powerfully and win so often, why does everyone else need to make such a racket? (no pun intended :D)

Apparently it makes respiration more efficient. Exhaling and inhaling deep breaths, so the story goes.

Serena Williams: UHHHHHHHHHHHHHHH!

Venus Williams: MU-Ahhhhhhh!

Ana Ivanovic: Ah-Uh-Huh!

Dementieva: WUAHHHHH!

Anyway, I don't think it's useful, but I think it's good because it shows you how hard they're working.

it allows a release of oxygen from the lungs, allows for the body to recover a little bit from each shot, therefore making the next shot easier and helping to keep a player from running out of breath after the end of a point. it also allows for the muscles to relax.

Let's think, if you're swinging your arm, so hard that you hit a ball almost 120 MPH, wouldn't you be grunting or making some noise?

It's a grunt, not an orgasm. It's an exertion of breath. It makes them hit harder.