If your posture is sloppy and off-balance, then chances are your shot will be sloppy and wayward.
When I watch pros on television address the golf ball, I see precision and attention to their posture. When I see recreational players address the ball, I see quick and sloppy with little, if any, attention to their posture.
Balance, athleticism, proper angles, and tilts are the key elements of posture. If done correctly, you are making it easier for the clubhead to do its job; strike the ball with speed and power.