Last updated on 26-Feb-2010 09:37:35 AM
Hi Toh,
There are a couple of reasons. First it can be a part of the follow through of the serve to get you body into the action.
Secondly it was definitely used as a distraction to the receiver. It can be used to disguise the sound of the ball hitting the bat which helps your oppoonent to tell what type of spin is on the ball.