For the same reason they stroke into the air 6 times before getting down on the shot. Then, they get up and air stroke 3 or 4 more. You should re-chalk after 7 or 8 air strokes. Air turbulence may have removed some chalk. Tap the spot on the cloth where you want to park whitey, even if it's a 6" draw shot or 6" follow. You'll look like such a pro........cue ball on a string sort of thing....master of the felt....the Sultan of Simonis.
If you shake it more than twice then your just playing with it.