They help players reach potential quicker per rating and have a in game modifier specific to the ratings applied in that scenario . Thier maximum influence would be 20% when compared to 30% coordinator and 50% for the HC.
Help your rookie qbs development of accuracy or distribute ratings to compensate for a weak spot in your hierarchy of coaches.
Imo the effects are minimal and I let the AI do most of my coach moves. If I do promote or sign a coach I look at that stuff to influence my decision .