It's long been recognised that a good pre-throw position is fundamental to a good serve and this simple and well known progression helps a player isolate this to develop a stable elbow at the end of the pre-throw position. Throwing from a consistent elbow position enables the racket to follow a consistent path and trajectory with subsequently a more consistent contact point.
Angles at pre-throw should be 90 degrees elbow to hand and 90 degrees elbow to trunk +/- 10 degrees.
Finish in a tip-toe stance (back foot) also begins to develop the necessary body-work (involves lower body - foot, knee, hip) for a serve whilst maintaining good balance and stability through contact.
Progressions can be used when players are further away from the final skill, corrections when they are closer to the final skill.