When an author tells a story just right, the imagination takes over -- and although the eyes take in each word -- the mind visualizes the story internally and lets it flow. Have you ever had this happen? Where you became so enchanted you can't put the book down?
While understanding is important to non-fiction writing, only fiction lets the mind take this extra leap to make the author's story a personal experience. It's similar to watching a movie or seeing a play, where for a few hours, you become emotionally involved with what is happening on the screen or stage.
If this has happened to you, have you ever stopped to consider what a wonderful process this is? I'm not sure what exactly happens, but I know the imagination is a powerful force to make the author's world seem real.
Even if you don't read scifi or fantasy, it is a magical moment.