"The best psychologist in the world is a square clubface at impact," - Ben Hogan
To be a consistent ball striker, you should strive to swing with a square clubface at all times, unless, of course, you are trying to intentionally curve the ball. Golf is a hard enough game hitting the ball straight, let alone fighting an open or closed clubface at impact.
Watch a good player swing and you almost always see a smooth, rhythmic start to their backswing. So what starts the club back? If you look at the swing as a series of circles, the clubhead makes the biggest circle, followed by the circle the hands make, followed by the circle the body makes as it turns back and forth. So since the clubhead has to travel the farthest, you want to start the clubhead back first.
Where a lot of higher-handicappers go wrong is that they roll the clubface open as they start the swing. This drill teaches you to keep the clubface square.