When deadlifting, after the bar passes your knees, you want to drive your hips through to finish the lift. Not over extend your back to finish the lift. The rack pull can shorten the deadlift motion, to really focus on the glute driven lockout. Then, when you go on to the full deadlift you pull the bar past your knees and WHAM your hips drive through, almost "automatically."
If you injure your hamstring you can do partial deadlifts, with the rack set to whatever height you can comfortably lift from.
If you start above your knees you can really overload your upper back and traps.
For lots of guys, what you can pull from mid-shin, you can pull from the floor. Only pulling from mid-shin saves a little wear and tear on the back.
When you're new to lifting, and you don't know the difference between rounding your back and arching your back, or how to really set up to pull, rack pulls can be an easier way to learn.
The key to carry over from rack pulls to deadlifts is to make them "feel" like deadlifts. Set up the same, don't make the rack pull into its own unique lift where you just jamb your knees under and hitch the bar up.