The biggest advantage to a short story is spent time. A novel, even if you are fast, usually takes a while. Months, in my case, though I once did a movie novelization in eighteen days, that's a cheat, 'cause I had the script.
You invest a fair amount of energy in a novel, and if it drops dead in the starting gate, you worked a bunch of time for naught. That stings. (Wait. I should qualify that: You might not make any money from an unsold novel, but the process of writing it is worth something. You can learn a lot from the doing of it, regardless of whether it sells.)
A short story might take a couple days, and in my case more than a few times, a single session at the keyboard only a couple hours long. It doesn't fly? So what? It didn't cost much, and you could stick it away and maybe turn it into a novel someday. Or put it up on Amazon.com as an electronic version.
Short fiction is an exacting form. No room to mess around, you have to keep things tight and on-point and it's not a marathon but a sprint. Pacing is entirely different for two thousand words than it is for eighty thousand words. You can't cover much–plot, setting, character, pick one, that's about it.
Um. Now and then, I do get the urge to do a short story. Usually this is when I'm up against a deadline and don't have time to be screwing around. A lazy writer will find all kinds of excuses not to write–the lawn needs mowing, the house needs repair, and even writing something other than what you are supposed to be writing will do. Busman's holiday.
Sometimes, a story just pops out of the blue, as happened with the Roy the Demon series. Got the idea for one, did it, and then there came a whole bunch more ideas until I was done with Roy, at least for the nonce.
Sometimes, the urge to write a short tale is externally-driven. Somebody sends me a note or calls me, they have a theme-anthology going, would I be interested in doing a piece for it? Sometimes the theme resonates. Conan Doyle meets H.P. Lovecraft? Sure. An urban magic thing? A theme line that starts, "There were rats in the souffle again ...?" Yeah, I can do that.
Back when Dean and Kris were starting Pulphouse, they kinda shamed me into writing for them. No, no, I said, I don't have time for short stuff, I'm up to my eyeballs in books!
C'mon, c'mon, you can take time off for a short story, Kris said. Guilted me into it, because writers are mostly lazy and always have more time than they admit to having, and she knew that, being a writer herself.
The result was that I wrote several wild-hair stories for Kris and Dean, at Pulphouse, then later when she was editing F&SF and because she prodded me into it, the best–for want of a better term–short stories I've written came to be. "Willie of the Jungle?" Still funny. The one about the two old hippies in the hot tub when reality breaks down? The guy whose house keeps changing back to the original color after he tries to paint it? Signs of a warped mind, sure, but fun ...
Um. Anyway, when I saw the Fiction River announcement on Facebook, I dropped them notes razzing them about getting back into editing, likening it to drug addiction. They razzed back, allowing as how they would hit me up for a story, and I waved them off. Don't have time for short stories, I said. Echoing a long-ago conversation.
The upshot of all this? I got an idea for a short story. Cranked up the WP, wrote a page or so last evening, and finished it this morning. If they ask? I got one. Another wild-hair piece, off-the-wall, but that's how it works around here ...