The purpose of this drill is to make sure you are bringing the ball straight back. As a pitcher, you are suppose to show the ball to the shortstop. In this drill you go through your basic mechanics, but you release the ball behind you to your partner who is standing or crouching. You release the ball as your arm is on the way back up. Your partner is only suppose to be a short distance away, as the ball isn't suppose to go far.
This drill benefits pitchers by making their mechanics better. It also teaches you to bring the ball down then back up, unlike other positions where you bring the arm straight back towards your ear. This teaches you to bring the ball straight back behind you.