Over the past several days I've promoted the Indies Unlimited Flash Fiction Writing Challenge. Now that a new writing prompt has been posted, beginning a new cycle of posts, voting and the announcement of the winner, you may be wondering why I'm pushing it so hard (and will be promoting some other writing challenges that I regularly participate in).
These sorts of writing challenges are a way of "exercising our writing muscles." When we take a prompt someone else gives us, whether it's a word, a picture, a song, or something else, and write a work of fiction inspired by us, we have the opportunity to practice several skills. First, these exercises generally have a short turnaround time -- the Indies Unlimited writing challenge gives you three days, with a firm deadline each Tuesday, and most others are in online communities that move on to other things after a day or two. This pushes you to get moving, which is useful for the endless ditherer who will spend "all morning taking out a comma, and all afternoon putting it back in." You have to find an idea in the prompt, develop it into a story (or at least a scene of reasonable size), write it, proof it and post it, all in a very few days.
Second, writing to a prompt is a great way to move outside your comfort zone as a writer and tell different stories. Almost every prolific writer has something he or she keeps coming back to, whether it's a theme or a plot pattern or a historical event. If we as writers are not careful, we can often end up rehashing the same basic story over and over. Having to write a story to a random image or poem or other prompt can get us out of a rut and into exploring fresh possibilities for theme, plot, characters, setting, etc.
Third, because these exercises generally have fairly low word limits (sometimes strictly enforced, as is the case with IU, and other times more loosely by how much one can write in a combox), it helps writers focus on sharp images. There simply isn't any room to develop a complex backstory, so you have to get enough across that the reader can understand and appreciate it quickly.
Finally, because these writing challenges are almost always on a blog or other form of social media, the interactive nature of them helps writers connect with a larger community of writers. Writing is such a solitary task that it is far too easy to get isolated.