The major thing that calculating your efficiency does is allow you to determine which plays are effective and which are not. This can vary from formation to formation and even from side to side.
How effective is your play calling? This is something that every coach needs to look at when the season wraps up. The issue is that this can be very difficult.
There are a variety of different ways to measure the effectiveness of a play call. Some teams go based solely on yards gained. While this can be an effective measure in most scenarios, there are lots of special circumstances (like the Red Zone, 3rd and 1, and any 4th down play) where this is not an effective tool to measure if a play was effective. In these situations a first down is the goal of the play, sometimes it might mean making a yard, other times it might mean making 10 yards but the measurement is still the same, first downs.