Why do I need a work permit?

I see you're in San Jose. Some states are different, but here in California, we have labor laws that are designed to prevent employers from taking advantage of underaged workers. I assume you're in school, so the employer has to consider this when hiring you.

You have school hours, you need time for homework assignments, sleep, meals, breaks, etc. The laws are in place to protect you from abusive employment behaviors.

When under age a work permit ensure you are not truant and are attending highschool. You are not permitted a permit unless you are in school and the school has the descression to approve or deny the request. See https://www.cde.ca.gov/ci/ct/we/wpfaq.asp#Q1 for further information about permits.