Why do you need to hit the ball at the top in table tennis?

I think, from a physics perspective, you should hit the ball at its peak because that's where it moves the slowest.

When a ping pong ball follows a projectile trajectory, its vertical velocity is zero at the peak height. Although its horizontal velocity remains constant, the overall speed is slowest at the top.

Hitting the ball when it's at its slowest gives you the best opportunity for timing.

Here some good  reference on projectile motion.

@roel  How about the spin of the ball? I also agree that the speed of the table tennis ball is slowest at the peak of the trajectory. But speed and spin are different. Is spin also the slowest at the top?

vanreyes,

From observation, the spin of the ball is also lowest at the top of the trajectory. You can hit heavy underspin with a powershot provided you perfectly timed it at the top of  the ball.