Question

Why should you refine your project before giving it to your teacher

Answer 1

You should refine your project before handing it in or turning it in to make sure you haven't made any mistakes that you haven't corrected beforehand. It also helps you refresh your memory about what you wrote or have made and in turn, if you present, it gives you a slight upper edge in presenting since you have a slight feeling of what you can say beforehand.
Basically, correct any mistakes, make sure the project is in pristine condition (so you can earn more points), and to refresh your memory in what you're going to say (if you're presenting).

Answer 2

So that your teacher sees your learning and not to lazy to not refine your project