Speed Fiction is a quick, literary one-two punch. Donald Barthelme was able to effectively tell stories with just a few paragraphs, and Douglas Adams liked to throw in a 2-3 sentence chapter or two in his works. It could also be compared to a lot of Guided By Voices' songs, which often would drop traditional structure to consist of nothing but a 30-second chorus. It's deconstructionist, postmodern and (arguably) a really cheap shot; writing corroded down to bite-sized pieces for the texting generation.
Personally, I like the challenge of restrictions. I think it takes effort to effectively summarize a thought in 140 characters for Twitter, and I like the idea of trying to develop a story snippet in a few hundred words.
Notable: Speed Fiction isn't real. It's not a thing. There aren't actually guidelines to follow. I shoot for 250-300 words or less, but more or fewer words makes no real difference. It really boils down to trying to achieve your intended result in as streamlined and condensed a manner as possible.
Also, it's a great exercise in editing. If you write something that is, say, 500 words long and you want to fit it into half the length, you'll have to struggle with re-expressing yourself, re-organizing and just flat-out trimming the fat of it. And if you look at it purely in that light, Speed Fiction 'rules' could also be restructured as an effective editing tool for _any_ piece of writing.
Or to write a story about Robots. Y'know, whatever.