I tend to write the story first then figure out the world by seeing it through my characters' eyes. However, my characters' prospective is limited by their superstitions and, furthermore, they lie. However, as their creator, I get to know the truth behind their lies, but sometimes, it takes me two hundred typed pages to learn it. Then I need to rewrite the whole thing. I am not the world building type of writer, but it seems like it would save me a lot of time.