The purpose of writing the proposal is twofold. First, it helps you think through your ideas about your paper. You can try out your tentative ideas and discover ideas as you write the proposal. Often, writers find that they need to get ideas laid out on paper to be able to work with them effectively. Second, by explaining your tentative plans to me, I can help guide you toward a more successful final paper.
"How do I know what I think until I see what I say?" --E.M. Forster