There's a lot of benefits you get by mentoring. When you have to teach you're forced to think more deeply about problems, see it from different angles, and revisit questions you had when learning but tabled at that time. That last one is pretty common. There's always questions you have when learning that you have to avoid because you don't have the knowledge to answer yet, because they would take you down too deep a rabbit hole. But later you have the experience to navigate those problems.

Personally, I've learned a ton while teaching. It's given me many new ideas and insights. But I'm obviously not alone in this. Even Feynman talks about how teaching results in better learning