This is contained in the other responses, but to make it explicit: another reason is to learn from somebody better or more experienced than you. I'm currently

This is contained in the other responses, but to make it explicit: another reason is to learn from somebody better or more experienced than you. I'm currently a founder of a company that is likely to have a pretty good outcome in the next not-too-long. I've learned so much about what I don't know, that I've determined my next gig will be working for somebody who can mentor me and help me climb to the next level of competence.