When you hit a ball, your eyes will instinctively follow the ball's path until the moment of contact (even if you can't really see the ball traveling that fast). Keeping your head still throughout your shot will help you maintain better balance. This is particularly important on shots close to the net, especially when you have to run up to reach the ball. It's very tempting to raise your head just before you contact the ball in order to watch the intended target. This typically results in poor ball contact and a loss of accuracy.
If you can keep your head still through the entire swing, your shots will be stronger, more consistent, and more accurate.