When someone writes a story, something is being invented. It is not merely a narration of facts, since otherwise it would not be a story at all, but a history, or some other kind of account regarding the world as it is.
Nonetheless, there is always something in common with the real world, or something implicitly supposed to be in common with the real world. Thus for example The Betrothed presupposes and sometimes mentions actual facts about seventeenth century Italy, even while including an invented narrative about individual persons. Similarly, the film Interstellar presupposes and sometimes mentions various scientific facts about the universe, even while adding various other things which almost certainly cannot exist in the real world, like time travel.
It is not difficult to see that it is essential to stories to have such a background in common with the real world, for if there were absolutely nothing in common with the real world, the story would be unintelligible. Among other things, a story must follow the laws of logic, at least most of the time, or it will be impossible to understand it as presenting an intelligible narrative. Consequently, a story will make sense to us insofar the background, real or supposedly real, makes the invented narrative a plausible and interesting one. Thus Manzoni’s novel must present a narrative that seems like a possible one in the context of seventeenth century Italy. Likewise, if the background implies that the invented narrative is highly implausible, the story will not make much sense to us. Thus, for example, while I enjoyed most of Interstellar, my experience was somewhat spoiled by the addition of time travel, and this generally tends to be the case for me when stories involve this particular idea. This is largely because time travel is probably logically impossible. To the degree that other people do not think that it is, or do not feel as if it were, it is less likely to disrupt their enjoyment of time travel stories.
The result of all this is that stories are one of the most effective ways to convince people of things. When we are giving our attention to a story, we are not in the mood for logical analysis or careful thought about the precise nature of the real world. And yet, in order to understand the story, we need to implicitly distinguish between the “real background” and the “invented narrative.” But in fact we may not be able to draw the line precisely; if someone does not know the details of the history of seventeenth century Italy, he will not actually know the difference between the things that Manzoni takes from the real world, and the things that Manzoni invents.The result is that a person can read the book, and walk away believing historical claims about Italy in the real world. These claims may be true, but they might also be false. And this can happen without the person having any explicit idea of learning history from a novel, and without noticing that he has become convinced of something which he previously did not believe.