HN has a slightly nerdy preoccupation with theory for its own sake, but a solid understanding of theory is immensely useful, particularly in jazz. You don't
need it per se, but in an awful lot of situations it's the difference between desperately faking it in the hopes that no-one will notice and confidently choosing from a broad palette of options that you know will sound good.
When taught properly, modern jazz theory is a clear and pragmatic aid to improvisational fluency. The time investment needed to get to grips with theory is really quite modest in the overall scheme of learning to be a jazz musician.
