the picture doesn't show for me but Sly's pretty much right, it's a momentum thing. It helps bring the body around to an otherwise unusual position, not unlike the front step's role in swinging a bat.
Kerry Collins always did this. I don't remember Beuerlein. Lewis did it, definately, and Craig did as well - but Craig was always inconsistent. Not only did his hand placement change, but his drop spacing was different from snap to snap, where he helf and released the ball often changed as well. It's possible someone could have changed and improved his throwing motion had they made it consistent, and that could have led to an increase of arm strength - more efficient use of strength. But consistency is generally controlled by the person, not the coach.
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but that's neither here nor there.