Music Theory is to Music what Physics is to the Universe; it’s an explanation for the way things work. Most of it is not theoretical; it’s something improvisers and composers put into practice every day. It grows over time to explain innovations musicians make in practice, not the other way around. Musicians rarely decide a new rule and then check to see if it sounds good.
As Mark Levine says, ”Theory is not the Poetry of music, it is the ABCs”. Some people think that if they learn theory (how music works) that it will make them play or compose more mechanically or conventionally, with less soul. That only happens if you barely learn theory, or if you barely practice scales, or keep doing it after you got it. It doesn’t happen if you internalize it, absorb it into your being, and move on.
Do great poets write more mechanically if they learn the alphabet, grammar, spelling, vocabulary…? Are painters less creative if they know Color Theory? Even folk guitarists learn what a I-IV-V is through practice. Do they become more or less creative when they discover, say, secondary dominants, either through records or a teacher?
Great jazz improvisers are not thinking theory when they play. They’ve used it to train their ears, then they forget about it and play by ear, just like a blues player, but with a much expanded vocabulary. Great poets don’t have to think, ”Where is the verb and noun in that sentence?” They’ve internalized it so they compose sentences ‘correctly’ by nature, and they still know how to break the ‘rules’ effectively. Dylan says “Ain’t” all the time, and Charlie Parker didn’t need permission to play b5s on I chords.