Keeping your head directly over the ball during the backswing causes you to shift your weight forward, instead of loading up on your back leg, causing a slice or hook
Think of your body as door hinges and your arms extended out as the door, when the door opens there is no roll in your arms, the toe of the club starts pointed up and goes back and through the same way, this is how your backswing should function
Practice this hitting under a rope drill to help keep your hands forward at impact and avoid an early release of the clubhead