Music theory is something that no musician can do without. It is impossible to learn how to play the piano if you do not know the basic foundations of music theory.
Just like, for example, you will not be able to read your favorite books if you do not know the letters and the rules of the language.
Theory is the language in which musicians speak to each other. Knowledge of the basics of theory opens up a wide field of musical activity - reading notes and performing musical works , analyzing and understanding music, creating your own musical works and the ability to write them down and so on.
In the classroom, the teacher uses textbooks on Bastien's Theory of various levels. In each lesson, students are given homework from the textbook and checked in the lesson.