Every second we receive huge loads of information from the world we live in but only a tiny fraction of it reaches our conscious mind (the set of rules that determines what kind of info goes there is called "gates of perception" and is adjustable), the rest goes to our subconscious mind and gets processed there in ways we have no control over. Things we forget also end up below once they have shifted out of focus of our conscioucness. That lot includes books we read, movies we watch, our daydreams, and most of events we live through.
Sometimes, the things we lost to the ocean of subconsciousness resurface as new ideas, discoveries, virtual simulations of people and worlds, dreams, fears, etc.

This is where GWI comes from as well. I don't consciously create the story, I just watch it as a blurry, dreamlike movie, decipher it for you, and sometimes fill the gaps. There are lots of such movies in my ocean, so many that I can watch a new one every evening before falling asleep. Can't help wondering how all those simulated worlds and characters fit into one tiny human brain in a body that lives on 1200kkal a day. And there are minds much more powerful than mine. We humans are amazing beings.