According to the latest issue of New Scientist (Jan 17, 2009) 'Novels help to uphold social order'. Priya Shetty asks 'Why does storytelling endure across time and cultures?'
She goes on to look at the results of a study into the way people respond to Victorian Literature. The study came to the conclusion that literature could condition people to fight against base impulses and work cooperatively. In the study people were asked to define novel characters and describe their personality traits. Self seeking ambitious people were villains and conscientious, supportive people were heroes. While characters like Heathcliffe were seen as combining both good and bad qualities. The point of it was that for a society to survive the cooperative types who worked for the greater good had to outnumber (triumph) over the greedy types.
If you look the epic fantasies you see the same theme played out on a grand scale. The villains are powerful and totally evil, conversely the heroes are often the little people who seem to have no chance, but find greatness in themselves. Like romances with their 'happily ever after' endings, quest fantasies enter into a contract with the reader who knows good will ultimately triumph over evil.
If people didn't have a need to repeat this scenario with the pay-off the underlying message -- you can make a difference, no matter how small and unimportant you are -- then they would not seek out the traditional fantasy.
The underlying theme of classic science fiction is similar. Instead of using magic to battle evil, the SF protagonist uses intellect and logic to make sense of the world/universe/aliens, ie. to battle ignorance.
The genres we choose to read reflect our world view and the way we interpret the world.
Story tellers serve a purpose!