Female tennis pros have been trained to grunt like that in order to get that last umph of power. They’re not competing for Miss America.
I mean this seriously ... who the heck cares what Serena or Venus Williams think?
You're wrong. First of all it's not a "grunt" but a full out scream. Second, a simple forced exhalation of air while striking the ball works better. Screaming actually takes more energy then a true exhalation.
They scream because it masks the the sound of the ball hitting the racquet (which is actually very important to judge the ball spin) and because they think it's intimidating.
The screeching should be banned.