The single best and most useful thing about flash fiction is that it forces authors to be mindful of exactly what they put on the page. You can’t be wordy in a flash fiction or spend time on unnecessary characters or plot tangents. You simply must say more with less and that requires you to tighten, which often leads to bettering, your writing. Every sentence, every word must be carefully chosen, and less wordy writing is often more impactful anyway. This helps you learn how to edit your own writing, which is a skill all good writers must develop.
You can use flash fiction to practice an area of your writing you consider your weakest. For example, you could do a flash fiction consisting of only dialogue. Or you could work on setting a mood and atmosphere from the first sentence. It also gives you a satisfying sense of completeness if you have a difficult time completing projects, and gives you more work to submit to publications. This will hopefully build up your publishing credits without having to sink as much time into writing longer short stories or novels.
Like short stories, flash fiction can also serve as the beginning or springboard for considerably longer works. Characters that come to you in a flash fiction might intrigue you and make you want to write more about their world. It can inspire you to create more, or capture a fleeting bit of inspiration that wouldn’t be suited for a longer work.
So how do you write a flash fiction when you even struggle to prevent short stories from becoming novellas or novels? The key is to focus on a particular snapshot of a moment in time rather than trying to tell a complete story. Rather like a written picture. You generally want to focus on just an element, an emotion, a brief occurrence, a character or two. You can write just a beginning or an end, or even capture the middle of a story. Flash fiction lets you break a rule or two.
Flash fiction is a great type of fiction for developing your skills as a writer, perhaps the most useful. I didn’t even know it was a thing until my creative writing adviser in college had us do flash fiction exercises, and I’ve been appreciative of it ever since. It’s sure helped me. So if you haven’t ever tried it before, or find it difficult, give it a try. It’s an excellent challenge for writers that only stands to improve your writing.