The kind of fantasy and science fiction that I like describes a possible world as it were from the inside (i.e., the sentences are to be interpreted relative to that world, as if that world were actual, in the sense of two-dimensional semantics—this makes it possible for the stories to have alternate origins for "the human race" and so on). Some of these possible worlds are fairly close to ours (realistic kinds of science fiction) and some are quite far from ours. Besides the kinds of values that every kind of literature can have, such as giving us a richer picture of moral deliberation, imaginative fiction of the sort I like also performs a devotional service—it gives us a richer picture of the power of God. There perhaps are no hobbits, probably there are no vast plasma-based intelligent beings in the sun, perhaps we do not live in a multiverse, almost surely there are no vampire-like unconscious but sophisticatedly cognitive beings, and probably God did not become incarnate as a lion; but all these things might have been so, by the power of God.
That does not mean that the fiction has to be overtly theistic or by a theistic author. Any picture of a genuinely possible world is a picture of a world in which God would exist, since God exists necessarily, in all worlds (and in the case of "God", the two-dimensional intension is constant, so we don't need to distinguish between conceivability and possibility). If the story is not compatible with the existence of God—for instance, if it contains a story of the ultimate origination of the cosmos incompatible with theism, or if it contains innocents suffering for eternity, vel caetera—then the story fails to describe a possible world.
Personally, I am made uncomfortable by imaginative fiction that does not describe a possible world. Besides rare cases of stories that appear to be clearly incompatible with theism, an offender is time travel stories that often violate metaphysical strictures against causal loops and circular explanation. I was also made uncomfortable by a Greg Egan story where mathematics itself is changed by human activity. (I think I am also made a bit uncomfortable by stories that strongly imply that what is happening is in our world—this world we live in—whereas the content of the story is metaphysically incompatible with how things are up to now. For instance, stories that give an alternate account of how "we humans" came into existence. But that is easily taken care of by reinterpreting the story without the rigidity of "our world"—that's what two-dimensional semantics is for.)