The posture, as with the grip, is crucial as it maintains the correct angle of the spine on the backswing and throughswing and in turn controls your swing plane. If this is not performed correctly you could injure your back.

The idea of the posture is to find a position which will not only keep you well balanced but will be one which allows you to generate a good solid swing around your body. The Posture also determines how far you stand from the ball.
There is a brilliant Practice...