To keep your story in focus. In simpler terms, to keep me from wasting my time writing five characters I don't need, and from going off on a tangent about the CDC studying malaria victims in a comedy about marriage. A focused story has ten times as much power as one watered down by side treks. A concept is basically a general idea of what the story is about. Concepts are about three lines long.
Concepts are basically useless.
What you need is an effective concept. Effective concepts will not only keep your story focused, they are excellent for telling others exactly what your story is about. That well developed statement can help you sell your story. A fully developed concept creates an interesting question and a result. They divorce and become more miserable.