I should start by admitting that there's just a touch of trolling in that title. Music theory seems to be jet fuel for some composers, myself included, while serving as an obtrusive stumbling block for others, preventing them from musically expressing the elegance of their feelings.
Let's dive right into one of the biggest music debates of the current century. Music theory skews heavily towards European tradition. That sentence is doing some heavy lifting, but that's as simply as I can state it. American students have been taught for generations that theory begins with Bach and Rameau, and ends with the serialists (12-tone composers such as Schoenberg, Webern, etc.). I have great affection for Bach and his Baroque contemporaries, and nearly as much for Milton Babbitt. But there's something either willfully ignorant or insane in declaring, or even subtly implying, that our understanding of theory wraps up nicely somewhere around the time of Debussy's death.
Among many others, one of the most obvious criticisms of such a notion is the mere existence of jazz. With the modern advantage of looking back at the development and history of jazz over the last 115 years (give or take), we can see pretty clearly the ways classical music and jazz have relentlessly intertwined. I might be just cynical enough to suggest that putting the music of the great French impressionists and the early blues greats into a blender would result in something akin to jazz, if not jazz as we know it. The big band jazz musicians of the 1930s and '40s saw no conflict whatsoever in "jazzing up" arrangements of classical music. Bandleaders like Larry Clinton, Benny Goodman, and Glenn Miller thrived while often incorporating well known classics into their setlists. Then there were others, like Duke Ellington, and later Stan Kenton, who deliberately fused both genres, well past the Big Band Era, in an effort to push music as an art form to a more radical plane of reality. Naturally those efforts were considered with varying degrees of palatability. So, why do we so quickly categorize jazz theory and classical theory two different animals?
It is often said that the case for Beethoven as the greatest Western (hemisphere) composer of all time can be summed up with a single point; every time he composed a symphony, the western world's understanding of music was changed. That's not an entirely unreasonable assessment. At the most apparent level, Beethoven redefined what made a work of music a "symphony". He started messing with the form, the orchestration/instrumentation, and a host of sounds that were blissfully shocking to the audiences of the early 19th century. However, one could make a nearly identical argument about Miles Davis and his role in the music of the 20th century. Much like Beethoven, Miles Davis was a superstar in his own time. For a time that spanned decades, he created a new sub-genre of jazz with every album he released. While he had significant collaborators, Davis was at the head of the line when bebop, cool jazz, modal jazz, and fusion sprung into existence. This is the man who gave us Kind of Blue, Birth of the Cool, Sketches of Spain, and Bitches Brew. The Ludwig/Miles comparison fades when you consider Beethoven's 9th against some of the music Davis put out during the end of his career, but that's not the point. At his best, Miles Davis was not only creating new ways to play jazz, he was inventing the theory that facilitated their creation along the way.
As a composer, music theory helps me to successfully articulate my musical language. At the very least, I know where to start looking for the colors I wish to express, and I'm always striving to expand my command of that language. Yes, I, like countless music majors before and after me, studied Bach's figured bass in college. Did it make me a better musician? Probably, though reverse engineering the theory behind Stan Kenton's City of Glass album, or Weather Report's Heavy Weather (actual things I'll admit without shame that I did in my early 20s) unquestionably shaped me more as a composer, and I never took any classes that required me to analyze them. To clarify, I wouldn't be able to write the music I do without the vast knowledge of European music theory I was taught. I'm simply saying that as I developed as a composer, I discovered a world of sounds, and the means to create them, that I eventually harnessed into my own musical language.
Western theory, along with its corresponding music history, is incomplete. It's not because Bach, Brahms, or Mozart wrote music that was "missing something". The reality is that in the 21st century, we have immediate audio and video access to singers of Indian classical music, Indonesian gamelan orchestras, Tuvan throat singing, and a host of diverse sounding music from around the globe, which 100 years ago was veritably unknown to American musicians and audiences. Set any snarky notions of "wokeness" aside. No one is forcing you to listen to anything or to incorporate the music theory of another culture into your compositions. But you do yourself a harsh disservice by assuming you'd dislike the music of 4 out of 6 inhabited continents. This also isn't a case to arbitrarily appropriate everything you hear. This is where sifting through the nonsense comes in. Understanding the music theory behind sounds that are new to us helps us to appreciate the fullness, the effort, and the profundity of the music by people who may not look like ourselves. After all, how can you claim the integrity of your own musical language if you've never heard anyone sing and play a different language?
Finally, let us tear down musical elitism. That elitism has isolated music theory to European tradition, removes the organic emotional appeal that draws us to music in the first place, and it teaches us, absurdly, that the likes of John Williams, Danny Elfman, or even Prince are hacks who have used their skills to sell out. These are people who have used an erudite understanding of what makes music great to move us and connects us as human beings. I invite you to check out some of the wonderful, down to earth videos on YouTube by Adam Neely, Jacob Collier, and probably many others, who explain music theory in a way that makes a person (read: me) excited to try out theory concepts in my writing, or to look at something I learned in college in a way that's more fun and accessible. Keep an open mind and open ears. And remember, it's much more satisfying to break the rules when you know what the rules are!