Ursula Le Guin in her preface to The Left Hand of Darkness [1], describes Science Fiction as "descriptive." She invents "elaborately circumstantial lies" as a means of describing what she sees as some truth in our being. The full quote:
> I’m merely observing, in the peculiar, devious, and thought-experimental manner proper to science fiction, that if you look at us at certain odd times of day in certain weathers, we already are. I am not predicting, or prescribing. I am describing. I am describing certain aspects of psychological reality in the novelist’s way, which is by inventing elaborately circumstantial lies.
This is how I think about both science fiction and fantasy. Elements of world building are different, even within each sub-genre, but this element of incorporating elements that are inconsistent with our world to tell stories is common to both. It's also why the term "speculative fiction" persists: a category that subsumes sci-fi and fantasy.