At its best Science Fiction remakes reality in the way no other genre can. It allows us a glimpse of the future, of unknown lands that we could not have thought of before.
It expands our collective imagination in the way little else can. It is important to remember that everything begins as a thought in someone's mind. Everything starts out as a sketch, an outline on a piece of paper. This is precisely why a lot of the NASA scientists acknowledge the influence that Science Fiction has had on their work.
To read Science Fiction and Fantasy requires at some level a suspension of disbelief and buying into the world that the author sells to you but paradoxically Science Fiction also makes you question the basis of society and everything that you see around you. In a way it exposes one to the hypocrisies latent in human nature.
To those who make the argument that Science Fiction is not literature do yourself a favor and read something by China Mieville, Ian McDonald or Paolo Bacigalupi, George Orwell's 1984, Aldous Huxley's Brave New World. You will be surprised with the quality that is on offer and indeed it is far better than the tripe that they call literary fiction. I have consistently found the Science Fiction and fantasy award winners to be far more readable and far more enlightening than Pulitzer winners or literary award winners for which Science Fiction and Fantasy novels are typically not considered. I guess this is primarily because Science Fiction and Fantasy novels are at their core about ideas and how people cope with these situations that they are put into which is I guess what the best stories are about anyway.
But I guess the real reason I like Science Fiction and Fantasy is that nothing else fills me up with a sense of wonder and astonishment as a good novel does.