You may not need a coach. You may not even be "coachable", i.e. you may already "know everything." However, every top performer in the world - from athletes like Tiger Woods, to the President of the United States - has mentors, coaches and advisors.
In studies of successful individuals, certain characteristics are always found: They are always goal oriented, they are always very persistent, and finally, they always have a person or group of persons that they meet with regularly to brainstorm and learn!
Simply ask yourself: On a scale of 1-100, "how do I perform right now?" If your honest answer is less than 100 - a coach can help you. Period.