Good question. I can think of two reasons.
The first should be obvious. Writing flash fiction helps authors sharpen their writing by forcing them to choose precise words, to eliminate empty words, and to focus on the theme of the story to make certain every scene, every paragraph, every sentence, and every word is important to the end result. Otherwise, the authors may be unable to write stories within the demands of the word count limits of flash fiction.For those of you who haven't read it, here's the rest.
FYI: As of last week, I am the sole fiction editor for Apollo's Lyre.