SF isn't bounded by anything except the requirements of internal logic, as is good fantasy. And I think SF has the potential to have a greater effect on the world because much of it is realistic -- that is, we can extrapolate from the now into the vision of the SF story and see how we might be affected by change or how we must act to enact or avoid that change. Most of fantasy is about the individual or clan, whereas most of SF is about the human species; though perhaps told from the pov of an individual, what happens in an SF story is seldom limited to that one individual.
Future stories, for example, by definition are about change and the human species' survival into that future. Stories where people can travel from star to star are similarly powerful. Historical stories tell little about what is to come, though they can be about the moments when things changed (and that makes 'em appeal to SF readers). That's why alternate history is so popular among SF readers, or stories about inventors -- sure, I write mostly SF, but my next story (Plug: coming out next month in the Synergy SF anthology) is an alternate history about Cyrano. What makes it SF (though it's a story about faith) is that it's the moment when the world changed... though it's not our past.