Science fiction is not all alien planets and galaxies far, far away. Often, the strange new world at the heart of the plot is this one-- Earth. What unknown terrain could be more fascinating to us than what our own home might look like in the distant (or not-so-distant) future? What species could be more compelling than the one we might become? Through science fiction, we can explore how humanity might affect its own future and how that future could in turn affect us.
But it's not always about looking forward; future-Earth sci fi is often as much a commentary on our present as it is an exploration of our future. A popular plot concept is to take a current trend or issue, project a future in which it has become commonplace (or has run amok!), and examine how humanity might adapt to the changes it brings about. Looking at an issue in this way can sometimes offer us a bit of perspective when otherwise we might be too close to examine it objectively. (Of course, it can also be sheer escapist entertainment, but hey, that's fun, too.)