Jazz was a genre mainly taught as an oral tradition, and of course improvisation was at the heart of it. Musicians played what they thought sounded good. That still didn’t stop George Russell’s book The Lydian Chromatic Concept of Tonal Organization from becoming hugely popular and influential among jazz musicians in the ’50s. Even when performers have an intuitive understanding of music, they can still benefit from explicit discussion of theory.

Also, a lot of jazz musicians wanted to eventually learn musical notation at least so that they could write their own lead sheets for copyright-claim purposes.