If your talking about making into a movie, It's so people could see it how stephanie saw it. Her view became your view. Reading the book you could imagine many things for Bella but in the movie you see the only face for Bella, Edward, Emmett, etc. Also because the books were a hit then why wouldn't the movies be a hit? People would make a lot of money and those playing the main characters would be known world wide.
If your talking about the book it's a simple answer. See Stephanie had a dream which made her sit down and right it. She got engrossed with it and ended up making a whole saga. I mean it can happen. Plus writing is a passion to her. Maybe she just wanted to make a new era a vampires to.