A Short Film That Reminds Us Short Fiction is SF's Lifeblood
The link title pretty much says it all. In fact, short stories are the lifeblood of many genres, not just SF- where would horror and erotica be without short fiction?
Short fiction is an entirely different animal from novels. Some authors can write great short stories, but can't write a decent novel; other are reversed, and still others can do both with equal skill.
I used to think I was a better short story writer than a novelist, but lately I've had a hell of a time writing anything under 10,000 words (the dreaded bastard-length 'novelette', which has few markets). I'm not sure why I'm having such difficulty recently. Maybe my brain is shifting into 'long form' phase. Hopefully it will shift back soon, because I enjoy the unique challenge short fiction offers- with less space to explain, you really need to use your words as efficiently as possible.
In closing, here is some advice from a better writer than me, one who I think was a far superior short story author than a novelist (hmm, this might get me flamed):
Kurt Vonnegut Tells You How to Write a Short Story