Rack pulls and halting deadlifts are usually employed to manage recovery. The full deadlift becomes too stressful, so we break the full movement into two partial ranges of motion. However, they also serve to reinforce correct mechanics.
Halting deadlifts require a fairly horizontal back angle throughout the whole movement. Trying to cheat by becoming too vertical too soon will be rewarded with a nasty tweak. This guards against the temptation to hitch heavy competition deadlifts.
Rack pulls are much heavier than a deadlift off the floor. The heavier the bar gets, the more temperamental it becomes. If it is not directly over the middle of the foot or if there is any looseness in the system at all, the rep will not go.
The same can be said about pin presses. Here, balance and control become even more important because smaller muscles are involved. Any slop is instantly punished.
Even experienced lifters forget the fundamentals sometimes. Partials, traditionally considered an advanced training technique, are actually a way to return to basics.