Anything and everything someone encounters in life will always have a more complex system behind it than it seems. The equation 1+1=2, for example, took 379 pages of proof in the Principia Mathematica just to explain why the equation is true. Obviously, the average person does not need to know this information to function in their daily life, but a mathematician understands this proof as the foundation of all science and logic. Much like this, music also has a very complex language behind it that some find absolutely necessary to understand even the primary basics of music, while others question if it is worth the effort.
Music theory is how people have translated the way music operates into a written, digestible form. While this form still takes quite a bit of time to learn, it makes communicating ideas to other musicians much simpler. Instead of trying to describe a harmony by singing through each note, they can take a few terms and mash them together in a way that makes sense to others who know the language. Instead of telling someone to be louder during this part of a song, one can tell them to either be forte or to add a crescendo, removing vagueness for how someone should get loud.
But while music theory adds a layer of clarity in one’s attempts to translate their ideas, some may still be adamant about going the basic route of spelling everything out. But, whether some musicians know it or not, they already have some knowledge tucked away in their back pocket.
When someone mentions music theory, lots of people immediately think of the tonics, the scales, the harmonies, the dissonance, and all of the other fancy stuff. In reality, music theory is just anything that makes understanding music easier. This means pitch is music theory, notes are music theory, tempo is music theory, and rhythm is music theory.
Many people who want to pursue music as a career, or even as a hobby, may wonder if learning advanced material is required to be successful. The short answer is no, knowing music theory is merely a strong recommendation. Take Elton John for example: one would think one of the greatest musicians of all time would have to be some maestro, but he actually taught himself to play piano at the age of three without lessons and just happens to have an incredible knack for music. However, because music theory is like a language for musicians to understand each other, it can make it difficult when working with other musicians if only one knows the language. The knowledgeable party might have difficulty explaining their ideas in a way that makes sense to the less knowledgeable, while the less knowledgeable will have trouble communicating their ideas without just showing them. So unless they can find someone to help them translate their ideas efficiently, working with others may be rough.
To make a long story short, no, music theory is not required to be successful in music, but with so many accessible resources like YouTube and musictheory.net to teach yourself music theory, there’s no harm in testing the waters.