I started writing short stories in my novel universe because I wanted to know about the backstory of particular characters. It definitely helped me with that, and in so doing helped me see how to improve the novel itself. Then I found that trying to write short stories was really helping my ability to look at stories from a structural perspective - improving my craft, just as the folks here have said.
None of the stories I wrote in that universe have been published, and at this point I don't really expect them to be. This is because they are so integrated into the universe, and the universe is so large and complex, that they don't stand alone well. However, that experience did get me into writing other short stories, and that is where I got my first professional publication. I still think of the novels as independent of any reputation I get from short stories, but in the end it may help me get more attention as I go forward.
I'll just wait and see.