(What's the story behind this scene?)All writers have fantasy worlds they live in from time to time. For example, there are at least a dozen alternate worlds I daydream about when I'm exercising or taking a long drive. Some turn into books, and some don't, but I've discovered that all have provided me with ample material.
In addition to the novels I've worked on, I've done several short stories that I plan on one day turning into a collection I'll sell. Most of those stories don't exist as a completely separate entity, but rather as a subset of one of those fantasy worlds I've created. I'm sure I could make up a couple of short stories out of thin air, but why? The worlds I've created are ones I'm already familiar with, and that makes writing stories within those worlds much easier.
(Familiarity with what you're writing is the key)Within these fantasy worlds, I already know the base setting of the world, and even some of the characters. I don't have to create a whole new world out of nothing. There's also an element of fun in going back to revisit a world you used to know as opposed to making a new one. It's like going back to your old high school after you graduate, only this time you have a sense of accomplishment, and even arrogance, about how far removed you are now.
I've done short stories off of Salvation Day and Akeldama, well as a couple of novels that will never see the light of day but continue to provide material for reference in the future. I think it'll keep me going strong off of those worlds for several years, even if I tweak some things and the settings aren't exactly what they were originally. If nothing else, it eases the burden on my mind so I can fill it with new worlds for full length novels...thus leading to new worlds for more short stories. Everybody wins!