I have what I call throw-away stories. These are stories that I never write down. They are sometimes complex or simple. I run through them in different ways, living in one character or another. But they are not full and complete stories, like the ones I share. They are not crystallized, like the ones I write down, but remain fluid, mutable. Sometimes I come up with the greatest ideas through them - and I might adapt it and write it down for another story. But these stay in my head, and I live and breath them and forget the great scenes I come up with and start them over again when the story can't advance any further. They don't have to make sense or be plausible in any way.
What's the point?
Well, for me, there are several points to this. The first is getting to sleep. Without these stories, I would not be able to quiet my brain enough for somnolence. The problems and worries of my life would just run over and over in my head, looking for a solution. When I've worn such mental tracks across my neurons for the millionth time and still come up with nothing, I can force them aside with a throw-away story that does not have the pressure of plot development or character consistency.
Another is to keep the image-generator in my head going. I live these scenes. I can feel, hear, taste, smell, and see everything each character experiences, and it runs through my head like a movie that I am a part of. It is more like an experience generator - I can be the powerful magic user or the downtrodden waif, the lonely hermit or the hunted fugitive, the victim or the terrorizer. I can be man or woman or beast. I can taste the fear or feel it, inspire feelings of helplessness or be lost in them.
Another is kind of taking a vacation. Sometimes I don't want to think about a story that I am writing - there might be a knotty plot problem that I have not worked out yet, and again, my brain would latch onto that and not let go. There is no such pressure with these stories.
Another is to try out the flavor of a situation. Sometimes I see a great scene in a movie, or read one in a book, and I don't care for the way it ends there, so I build my own very improbable scene and try it my way. This is how I got started writing in the first place, by changing the endings to stories or movies that I like, but thought the ending should go another way. They let me use other people's characters that I can change to my liking, take a piece here, add a dash of something I saw or read long ago, and - Action!
I love my throw-aways as much as the ones I share with the world. And if they have enough potential, I do write them down and share them with everyone.