I am writing a novel right now and have a whole universe set out for it, but I only managed to work out all the kinks of my fictional reality my writing a selection of non-cannon short stories.
When ever I do something creative I layer it so a short store begins as a pre-draft then gets rewritten into a first draft and so on, with my novel I am doing very much the same process except I am writing and splicing many already completed short stories together. Its all the means to a greater end to me.
that's why you plan. but that all depends on what you want to do, if you want to write them at all. i find them way more satisfying to write because i can bust one out and post it and loads of people will read it. no one reads my novels.