Rack Pulls are a staple exercise among powerlifters from beginners just learning to deadlift to experience and elite powerlifters looking to get the most out of their training. There are two main reasons for including Rack Pulls in training and they are
Deadlifting with a neutral spine
Many beginners have a problem with maintaining a neutral spine position when they first learn to deadlift, without the ability to maintain this spine position the exercise becomes dangerous therefore Rack Pulls can be used as an assistance exercise to build a base level of strength through the back while also including mobility exercises within the training program to increase the safe range of motion for the spine. As back strength and mobility improve the lower the bar can be set for Rack Pulls until eventually range of motion and strength are achieved for dead-lifting from the floor.
Overcome your Deadlift sticking point.
For experienced powerlifters Rack Pulls are used to overcome sticking points in the movement and to improve lockout. When using rack pulls for this purpose the weight is usually a little more than what would be used in the main deadlift exercise and the bar height is set to just below the Deadlift sticking point.
When you can overcome both form problems (spine) and weak areas (sticking points) you will begin to see improved numbers when you deadlift from the floor regardless of your powerlifting experience.