There’s a screenwriting principle called Chekhov’s Gun that says if you show a gun on the table in act 1, you had better have used it by the end of act 2 (or something like that). The principle is that every time you introduce a fact, you are making a promise. The promise is: “Dear reader, if you will please remember this fact, I promise to explain why it is critical to the story by the end.” Readers don’t like being asked for favors, and they really don’t like promises being broken. So we have to be parsimonious with the favors we ask, and make sure we keep our promises.