Well, not just Sci-Fi, but fiction in general.
As I've been reading 2010:Odyssey Two by Arthur C. Clarke I've been rediscovering my love for Sci-Fi literature. The last Sci-Fi novel that was this good was Ender's Game by Orson Scott Card...and before that it was Dune by Frank Hebert. (Seriously, you should read these books... they are good literature no matter how snooty you are
My brother read almost all of the spin-off series to Star-wars... many take Star Wars and Star Trek as near religion.
The main benefit to these books is not just entertainment, relaxation and a sense of identity... though these certainly are benefits.
The main benefit to a good work of fiction is that it helps you to dream, to escape your boxes and to dare to imagine the impossible.
What could man achieve that he isn't currently achieving? What in your mind is commonly held to be impossible that you know is possible?
For me, I dream of cheap energy and abundant artificial intelligence. Never before has man been on such a cusp of free-time. We know how to capture the sun for energy, we know how to farm intensively without draining the soil of it's nutrients, and we have computers which can learn, work, and behave like very simple intelligent beings (often outperforming us at many tasks). Couldn't we live in a semi-utopia with the help of our technological advances?