They're different

Melanie, in StoryWeaver 4:

…consider that your main character does not necessarily have to be your protagonist. What?! Are they not the same thing? No. The protagonist is the character driving the plot forward toward achieving the story’s goal. But the main character is the one who dive (sic) the thematic message forward with an assault on their moral values or belief system and must ultimately decide to change or remain steadfast in their views, often in a leap of faith moment near the climax of the story.

Often these two dramatic functions are assigned to the same character, creating the standard hero.

Sometimes, assigning the roles of protagonist and main character to two different players enables opportunities that would never be possible through a standard hero.