What is the purpose of doing a rack pull or block pull instead of a full deadlift?
There are probably a couple of purposes:
It easier to add or remove weight plates if the bar is 12–14 inches off the ground.
You can work on starting the lift at the sticking point, if that's preventing you from getting stronger
The higher the bar is placed on the rack, the heavier the weights that can be used, so you can get supermaximal or isometric benefits.
If you have an injury that prevents full range of motion in the deadlift, using rack pulls can be an effective solution to training around the injury, and maintaining strength without aggravating the injury.