What is jazz theory for?

Jazz music theory certainly isn't the most important thing to play jazz music. Most of the times it even doesn't help playing since you don't have enough time to think theory when you are sight reading the chart on stage.

What then will help improvising jazz music?

Set your idol, and try to imitate them. In my case, Miles is my God and I even don't try to study him but just warship him instead. Once upon a time, I listened to his 1964 Lincoln Center recordings for one year, over and over, try to memorize everything what's on that two disk sets.

I had quite a few idols, not at once, but one by one. For performance, there were Wes Montgomery, George Garzone, Eddie Gomez, Pat Metheny, Michael Brecker. I transcribed them in my early days a lot. For composition, George Russell and Gy├Ârgy Ligeti.

Would theory help?

Unless you are a genius and can play absolutely anything by ear, not to mention photographic memory to memorize everything in one path as Wes Montgomery was, you need certain theoretical skills.

With this information above, you construct your improvised melody with Root, 2nd/9th, 3rd, 5th, and 7th, A Coltrane approach . If you are grooving, you are all set.

What is jazz theory for?

Deep understanding of jazz theory is very much needed once you wish to write your own piece. This is not like classical music which others just read notes you wrote. A composition that is badly notated with improper theoretical background only makes other players play your music badly.