Why? Just write whatever comes to mind! You may end up with a huge, messy, surreal ball of nonsense, but it seems like that sells these days, too....I'm finding out that I hadn't really ironed out a story, just a basic idea. So I'm constantly writing stuff, then being like "OK...why did you write that?" XD
Need to plan something, I suppose.
Sometimes it's not about the destination. A lot o the time, it's merely having fun with your brain while you get to where you're going. If you do end up with a work that you're sure wouldn't sell even if you edit it into something different, don't worry about it! At least you had fun! And you can pick through what you created later, when you're working on another piece. (I've had one idea for a novel that I've considered but never fully fleshed out. I'm thinking that I may be able to weave in some of the characters and plot elements from that story into the one I'm writing now, though. Nothing wrong with rescuing scraps from the garbage bin when it comes to writing.)