How many notes are there, what are their names, what are the treble and bass clefs, what kinds of notes exist, how do we write music down — all of this is what one needs to know in order to read a simple piece from a score. This is the very basics of music theory.
When I explain to my students why they need to continue studying theory beyond these basics, I usually play them a succession of random notes and ask if it sounds good. Their answer is always "no." Why does it sound bad? Because it is a disorganized array of random pitches, without any system to it, without any thought put into making it sound good.
Music theory as we know it has been developed over many centuries by generations of outstanding composers and theoreticians who devoted their lives to understanding and improving the writing of music. When they created the music we now admire so much, not a single note or rest was left to chance. They knew the rules of how to make music sound beautiful, as profoundly and extensively as their time period permitted.
Nowadays, when students learn to read music without learning theory alongside it, all they do is read the notes — without any understanding of why those notes are there and why they sound so good when played as written. They might notice patterns and have a vague intuition about certain chord progressions, but on the whole they wouldn't have a clue about why they are playing the notes they are playing.
Learning music theory opens up a whole new world. Students start to see the harmonies, how they interact and why; they see texture and structure, which notes are more important than others, and how music is organized. It helps them make the pieces they are learning sound better, become much better sight-readers, memorize more easily, and generally become better musicians.
As a professional musician with a doctorate in music, I can say with complete assuredness how much I appreciate the excellent theory, harmony, and analysis education I received in my youth — and that is what makes me so passionate about teaching the subject myself.
