Those coordinators that use grades do so to make Bowl assignments for their staffs. Wasn't always the case, but, for all I know these days, that could 100% of coordinators.
Some coordinators may have a performance threshold, above which any official may get a bowl assignment, even if his grades are not the absolute highest. There are cases where an official performs at a very high level, but there is that one guy, at the same position, that always grades out the highest. It isn't fair to deny a bowl assignment - or a top tier bowl assignment - to a guy that performs at a very high level, when that one other guy will always out-grade him by a few points.
Now, in some cases, that's their stated policy. In reality - and I have actually heard one say this - they are going to put people they want in bowl games (irrespective of grades). Grades from position graders for specific calls in individual games have been known to be changed by the coordinator, for the purpose of improving - or degrading - the evaluation of specific individuals, to justify bowl assignments and/or post-season evaluations.
Believe it, or not.