Music Theory is an important part of the foundation for any musician for several reasons.
Let’s pretend you had to give a speech in a foreign language. How important would it be to understand the meaning of the words? It would be impossible to give the speech with the appropriate inflection and pacing without having a thorough understanding of the meaning and structure of the speech and all of the words in it. Music theory, like language, enables us to understand the structure and meaning behind a musical composition.
Even more importantly, studying music theory and solfege greatly speeds up the result. Usually one third of the private lesson is spent on drilling the notes, rhythm and some theory, which leaves little time to learn new pieces. Since the basics of music theory and solfege will be learned during the theory lessons, the instrumental lesson will be fully spent in learning new repertoire and advancing the technique.