Actually, the bus runs both ways. Music theory is not a rigid set of rules that everybody (except folk musicians) has to follow. It's fluid and ever changing. It is derived from studying what real live musicians do, looking for patterns and principles, and trying to work it into some comprehensive system. For example, the first time a Dominant 7th chord was ever used (that anyone knows about), it was by Monteverdi an a choral arrangement of Summer is a-Comin' In, right at the end. The Dominant 7th chord contains a diminished 5th (in a G7, it's the combination of B and F. In medieval times, a diminished 5th was regarded as "The Devil in Music" and it was considered on the verge of blasphemy to write it or play it. But Monteverdi wanted a "drop the other shoe" effect at the end of the piece, so he went ahead and used it anyway. Critics at the time were outraged. One of them said "The human ear will never grow to tolerate such dissonance!" But . . . there are not very many pieces of music written within the last couple centuries that don't make extensive use of Dominant 7th chords. It's one of the Big Three basic chords in any key. "The Devil in Music" is the famous "flatted fifth" in jazz. Nowadays, it's pretty tame. Music theory adapts to what is actually being done.
I started singing and playing the guitar when I was 22, and I was a real musical ignoramus. I had to rely on other people or on chord diagrams in songbooks to show me what chords to play. Since my interest in folk music was really serious, I decided to study music so I would know what the hell I was doing. A few of my folksinging compatriots had a hissy-fit. "They'll ruin your creativity." "You'll be bound by all kinds of rules." "They'll stifle you." And all kinds of other dire warnings. But far from being stifled, studying music theory showed me what is possible. Within a couple of months I was working out my own accompaniments. Within a few more months, people were asking me what chords to use!
Sure, I could take a folk song and arrange it like a Beethoven string quartet. But that didn't mean I had to. My guitar accompaniment for The Three Ravens is a dead-ringer for a Renaissance lute accompaniment. But I accompany Down in the Valley with two first position chords and a thumb-strum-strum, thumb-strum-strum right hand.
I've always been curious. . . a very good folksinger-guitarist friend of mine was one of the most vociferous in trying to persuade me not to study music formally. He told me that he couldn't even read music, he didn't know how chords were constructed or what notes made them up, and he didn't know how the chord families related to each other. He had learned everything he needed to know from a copy of Guckert's Chords for Guitar without Notes or Teacher, and he was getting along just fine. I learned much later that before he took up the guitar, he had studied classical violin for nine years. What was he playing at, I wonder. . . ?