What do you know about Harry Potter?
How do you know if you're a wizard? You can't go to a doctor and ask for testing. You must wait. For years. But what if you don't want to wait? What if you're past 11? And you don't have a letter?
What if you need to prove you have magic blood? That's why we have quizzes, and this is for the sole purpose of the complaints stated above to fade away.