That's exactly the point. You can do that in your own head, but you can't communicate that to other people. For every amazing thing you've ever seen in a movie screen, imagine how much MORE amazing it was in someone's head before that. But you can't capture that in full.
And, much like the scientific method can take something that "everyone knows", and turn it into the basis for a branch of science, the process of getting this down in paper forces you to document all the nuances in your head, making that final idea even richer and fully consistent.
And, much like the scientific method can take something that "everyone knows", and turn it into the basis for a branch of science, the process of getting this down in paper forces you to document all the nuances in your head, making that final idea even richer and fully consistent.