For a story to be complete, the audience will need a personal view of the dramatic battle as well: that of the soldier in the trenches. Instead of looking at the Story Mind from the outside, the Main Character Throughline is a view from the inside. What if that Story Mind were our own? That is what the audience experiences when it becomes a soldier on the field: audience members identify with the Main Character of the story.
Through the Main Character we experience the battle as if we were directly participating in it. From this perspective we are much more concerned with what is happening immediately around us than we are with the larger strategies that are really too big to see. This most personally involved argument of the story is the Main Character Throughline.
From the Dramatica Theory Book