I've been (racing / autocrossing / driving at drivers' schools) for ten years. Why do I need an instructor?
Because our schools focus on learning, every student, regardless of experience or background, is assigned an instructor. Our instructors have a lot of experience and are there to help you improve, even if it is only in some minor way. Think of it this way: why learn something the hard way when there are instructors available who will give you that information for the asking? Unless you've already received a contract to drive in Formula 1, there's probably still something that you can learn about driving.