Why spoilers matter(I have a sense that I might have said this before, but never mind).
I was reading Sandman: The Kindly Ones recently. At the start of the book a prologue spoils the end. It then belittles the reader, claiming that the end was obvious, and they should have seen it coming. Well. While the ending made perfect sense, it wasn't the only ending that would have done so, and to know it was coming DID detract from the experience.
Listen, books and films, narratives of all kind, aren't ruined by being spoiled. There is still something to be had from experiencing the narrative no mattter how much you know, otherwise we'd never watch that thing again. Nevertheless, experiencing narrative for the first ever time is something that cannot be replicated. The joy of discovery, astonishment at twists and turns, even if they were narratively sign posted- just because something was likely to happen does not mean it will. Authors do change their minds as their stories go in an unexpected direction.
So spoiling does take a little from that- just because it was probable that a certain character might die, we did not know when, how, or who, and to have that taken away from us is a pity. I was fortunate enough to be unspoilt for planet of the apes, for an example of something its practically impossible to see without having being spoilt for, and it DID improve my experience.