Someone asked so I'm answering, though I don't usually explain my rules. Cuz first rule: there are no rules. This will be a five part post because I started writing out all the reasons why I think writing short stories is good for writers, and it got HUGE.
The short form teaches you to write. I'll get into this more in another post, but for now: creative writing and masters programs all around the world focus exclusively on the short story. They can't all be wrong. Many weeks-long workshops hone short story writing skills, too... what is it you all think they do at Clarion anyway?
Too narrow to my genre, speculative fiction (also the preferred genre of many of my readers). Many, many spec fic authors, I'm talking about the people whose names you know, and other people know, because they're on best seller lists, your Gaimans and your Scalzis and your Vaughns--and who's that guy?-- oh yeah, Stephen King, started out with the lowly short form. Speculative fiction in particular has a tradition of building careers through short story credits. I also have a good friend, a thriller writer, whose agent discovered him via a short story in an online magazine that pays its writers with a t-shirt. So even the little markets can be worthwhile.
It's not only speculative fiction and it's not only SFWA-recognised markets. Any writer who says the markets have shrunk, who claims there is no place for short fiction, is not paying attention. Just go to Ralans.com and you'll find a dozen places to send any given short story. Even erotica and romance are selling tons of short fiction online. And it pays. It may not pay a lot, but since when did we get into writing fiction to get rich?
I don't know about you, but I write because I have something to say. So tomorrow, we'll focus on exploring what you have to say and learning how to say it, via the short form.