What do courses teach that you genuinely struggle to learn just by gigging?

I’m not anti-course at all, but I keep going back and forth on whether I actually need one yet. Open mics teach me very quickly whether a joke works, but they don’t necessarily tell me why it didn’t, or what I should practise between gigs. For people deciding between structured training and just getting more stage time, what skills do you think are genuinely harder to pick up on your own?