Protagonist and Main Character are two separate terms that describe two very different concepts of story. Often they are mistaken for the same thing. The reason to split them apart is because it is more accurate to do so. The explanation is provided in this choice link.

You’ve probably noticed that we’ve used common terms such as Protagonist, Main Character, and Central Character in very specific ways. In actual practice, most authors bandy these terms about more or less interchangeably. There’s nothing wrong with that, but for structural purposes it’s not very precise. That’s why you’ll see Dramatica being something of a stickler in its use of terms and their definitions: it’s the only way to be clear.