There are advantages to not letting the play clock run all the way down to 1. If you let it run down to 1 on every play in clock killing mode, the defense knows exactly when to fire off the ball to stop your run game. If you mix it up, the defense has no idea when you're going to snap it, which keeps them on their heels. One of the great advantages any offense in football has is knowing when the ball will be snapped, while the defense doesn't. By telling the defense exactly when you're snapping the ball, that's a major equalizer. That may sound like "Football 101," and it is.