Written works of science fiction that take place in the future or in alternate versions of the past or the present*, in addition to whatever fictional characters may appear explicitly in the narrative, also implicitly create two others: the fictional writer and the fictional reader.
*Concepts we should never allow ourselves to think of as natural or always-already-understood or just-given are in italics.
The fictional writer: the story pretends that there exists someone who could be aware of these events that have not occurred yet or at all, who in that awareness could write about them (this is true whether the story is in the first person or not). The fictional reader: one can only read about events after they are written about, which can only be done after they occur, and these events have not occurred yet or at all.
(There is of course a sense in which this is true of all fiction; it could possibly be fruitful to pursue the questions I hope to raise here in all written works. But at this point at least it seems to me that no other kind of writing demands the pursuit of these questions the way sf does.)
The relationship(s) between these two characters, the relationship(s) between both of them and the work, and the relationship(s) between all these and us, the work's "non-fictional" readers, are one of the fundamental determining factors in everything that the sf work does, can do, seeks to do — and the converse: everything the work does not do, cannot do, does not seek to do. (One could also, perhaps better, put it in another converse way: that what the work does and does not seek to do is what determines these relationships.)
Some works address these relationships explicitly. Joanna Russ's We Who Are About To and Clifford D. Simak's City, for example, both insist that if there is a reader, it is certainly not us, even that we certainly do not exist. The Russ presents itself as a found document that manifestly could never be found, at least not by anyone human and probably not by anyone at all (and its fictional writer — or more accurately its fictional speaker — knows this, discusses it). In the Simak, the impossibility of the reader becomes apparent when the characters we are reading about discover that their past (in which we in our present are living) literally does not exist (this not to mention what the "essays" do to the status of the fictional writer).
There are other ways than to assert the impossibility of the reader. I think for example of Gene Wolfe's Book of the New Sun, which pretends, without telling us how, that texts from the future are as available to present-day translators as texts from the past, or Henry Kuttner's (and, probably, C.L. Moore's) "Mimsy Were the Borogoves," which begins by declaring the impossibility of describing the future from the vantage point of the present. Even such seemingly naïve (and, in sf, commonplace) devices as the appendices in Frank Herbert's Dune or Isaac Asimov's ploy of opening the Foundation stories with entries from an encyclopedia that does not exist yet even in the vastly future world of the stories themselves, stake out relationships between the work, these implicit fictional characters, and the actual reader that contour everything the work does and everything about how we read it.
But such relationships are staked out, in different ways, even in the remaining vast bulk of sf works that do not make any explicit issue of the fictionality of the writer and reader, that simply take it as a given (or pretend to take it as a given) that it is possible for someone to write about these events, and possible for someone else to read about them. And again these relationships countour everything about the work and our response to it. (Among many other things, consider the ways they affect the choice of verb tense, a choice whose ramifications are quite different in sf works than in others.)
These issues are inescapable in sf. It is perhaps (perhaps) not too much to say that the work's demand to deal with these issues is the larger part of why a writer might choose to write sf rather than something else, whether the writer would put it this way or not. These issues are implicit in everything that sf is, and in everything that is said about it. It seems to me that careful attention to them is vital if any critic's, any reader's, approach to any sf work is to have any chance of coming to terms with that work.