any strenuous repetitive motor action utilising the wrist, arms, shoulders, upper back (raquet sports, (drumming - I speak from experience) require the trunk to be as balanced as possible at the moment of most stress (the hit). The centre of gravity being the naval (or belly button).
It helps them stay balanced. Just part of the technique of the stroke really. You know, it's funny because my friend who doesn't play tennis asked me the same thing yesterday. He found it weird that they were sticking their hands out in weird angles too haha.