I got an idea for a new book on the way home from work. I think it'll be fun to write when I get around to it, but it's not ready to write yet. I have an idea, not a story--certainly nothing resembling a plot.
By the time I got home, though, I had worked out the main character's problems and how he's going to attempt to solve them. So I sat down and wrote 1,500 words. Now I'll set it aside for a few months or maybe even a few years while my subconscious works on the story and I finish other projects.
So why did I write those 1,500 words if I don't plan to continue right away? I do this a lot and it has several purposes. The main one is just to remind myself about the idea in case I forget. Also, gets my brain thinking about possibilities. And when I do come back to it, I have something to build on.
I'll probably throw out everything I've just written. But that's okay, it's just an idea.