Think of what the word 'exam' means. It's an examination to see how much you've learned during the past half of the school year. What have you learned? If you haven't done well, it helps the teachers know your strengths and weaknesses, whether you need more help and where. You can't learn much when you're tanked up with junk food. Establishing good habits when you're young will help you greatly if you apply it during during your adult years. You're welcome.