The shoulders serve as a guide to the golfer exactly where his shot will go. When a right-handed golfer is aimed properly at the target, his left shoulder will be pointing directly at the target. The shoulder also serves as a guide for the left foot. That foot should line up directly under the shoulder. If both are aimed at the target, the ball has an excellent chance of ending up where the golfer wants it to go.
To generate power in the golf swing, the shoulders must rotate. As the hips turn to trigger the swing, your upper body turns away from the target. As that happens, your right shoulder also turns and gets in a position to launch into the ball with force. This is how much of the power is applied during the golf shot.
Your arms supply the leverage when you swing the club. Your left arm must stay straight on impact and that's what supplies the leverage when you hit the golf ball. The power in your body is transferred through your swing and your straight left arm acts as a lever to concentrate that power in the head of the golf club.