I've sold fifteen (or so) short stories. Some went to pro-markets; some went to small. Some were flash, some were horror. Some were fantasy. Some were SF. One was a collaboration. Some were to invitation-only anthos; one was specially commissioned by an editor in a bad place.
And every single sale was valuable. Every one of them has advanced my career, even the sale that made me a mere $10 Canadian. Why? Because every story I have sold has added to my social network.
When you submit a story, it passes through the hands of slush readers before it reaches the editors with final say. Many of these slush readers will advance in their careers--and I know for a fact that yes, they remember your name. I just spent a weekend hanging out with a slush reader-turned-editor who remembered our first introduction, a story he rejected from his slush pile two years ago!
So there's the first seed your story plants in a potential network of relationships. Then you have the editors, who will not only be paying you and promoting you, but also shaping your work--and if they're good, sculpting and affecting your relationship with your words. Line edits from a good editor can teach you a lot.
And of course, those editors remember a writer, their work, their professionalism. With any luck they might ask you submit again, to another project, or they might pass along your name to other editors. When you make a good connection with an editor, you connect yourself to everyone that editor knows ... and sometimes that's a very large network indeed.
But the really wonderful connections you make, the ones that might surprise you with their value, are the connections you'll make with the writers who are published alongside you. A new friend was published in the same small YA magazine I was, and from reading my story, he realized our writing styles were very sympathetic with each other. He asked me to collaborate on a project together that will be really fun. That's an opportunity that wouldn't have existed if I hadn't sold that YA story.
I just yesterday received an invitation to submit to an anthology that came from a ToC-mate who is now editing. (That's the second time this has happened.) I just today received an invitation to a writing retreat that's being organized by a remarkable writer who shared a ToC with me in a very tiny, now defunct erotica market. At World Fantasy, an anthology ToC-mate shared tips about a new market with great rates.
And remember that tiny Canadian anthology with the wee paycheck? Every day, the people who shared the ToC of that anthology write me, email me, share their anthology invitations, share their good news about their promotions within the industry, hang out with me at conventions, and introduce me to their other friends: editors, agents, and novelists I admire in the field.
Let me reiterate: these are opportunities I wouldn't have had if I hadn't sold those stories. Going to conventions and doing volunteer work have certainly helped my career, but the deepest and most helpful connections I've made have come from the editors who've bought my stories and writers who have shared tables of contents with me in magazines and anthologies.
Your writing career will be different from mine. But I think most writers find that they do not become superstars writing short fiction. They find that writing short fiction strengthens their craft and enables them to build a strong network of friends who are at similar stages in their career. The skills developed writing short fiction give writers the muscles they need to pick their way up the rocky slopes of an ascending career. The friends writers make will help them over the roughest terrain--or if you're really lucky, help you cut a switchback here and there.
Story by story, friend by friend, the summit approaches.