We know SF, and are familiar with its history and its application. Fantasy, as a made-up universe, is similar, and plays a similar role.

When I began reading SF, critics spent much time and effort trying to define its role in literature. SF wasn't just a made-up universe, because, after all, that can be said of ALL fiction. SF, or speculative fiction, created an artificial universe in order to examine some issue relevant to the "real world".

For example, "Twenty Thousand Leagues under the Sea" was NOT science fiction simply because he had inserted a submarine into the 19th century. The book was more a psychological study of Captain Nemo, with slight digs against "the hated race" (The British Empire). His technical description of the Nautilus was scientifically rigorous, but it was incidental to the story. Now if Verne had speculated on the role of submersibles in modern warfare, how it might affect conflict and military strategy, THAT would have been true science fiction. Most of Verne's science fiction was not really about social themes, even though his science was excellent.

HG Wells, OTOH, cavalierly incorporated very questionable pseudo-science into his stories, such as Cavorite and time machines, but his stories were built around some contemporary political or social theme. "War of the Worlds", for example, was a not-so-subtle dig at British colonial aggression and imperial economic exploitation.

The critical interpretation of SF that most appealed to me back then was that it was an attempt to understand or predict the effect of new technology on the human condition. How will space travel affect history? What will be the social consequences of a vaccine against aging? How will those consequences change if that immortality drug is made freely available, or if it is restricted to an elite? How will a peaceful contact with benign extra terrestrials affect human history and development? How will abundant, cheap, clean fusion power change the world? How would a perfect telepath function in contemporary society, for good or evil.

The ideal SF tale studied the effect and result of one, or just a few, technological explosions on the world, or on a single human being. It was like a controlled experiment: Keep everything the same but change just this one thing.

In this model, the program of SF was to insert a technological innovation into society, or into our life, and try to predict the consequences. The new tech was not what the story was about, it was about us, and how we responded to it. It was an attempt to understand us, not the technology.

I believe that is the role of fantasy as well. And I don't believe I agree with Tolkien on that.