Books

Yes, absolutely. I can enjoy second reads, but discovery can be an important part of the journey. Will I get as invested in character A if I know in advance they're going to get killed off? If I don't get as invested, doesn't that lessen the emotional impact and thereby diminish the story?

Someone mentioned that we all know how The Titanic ends. But the fate of the Titanic was never more than a plot device, and the ship wasn't the focus of the story.

I was lucky to have been utterly ignorant about Sixth Sense - I hadn't even heard there was a twist ending, so I wasn't looking for one, and the ending was consequently both a surprise and so much more impactful than if I'd known how it ended.

I find it does. Part of the enjoyable tension of a book is not knowing what comes next, and the satisfaction of a plotline well-executed.

I remember seeing a study in which most participants expressed dislike for spoilers—but when groups watched films (or read stories—I forget which) with and without being given spoilers beforehand, the reported enjoyment of the groups that had been given spoilers was higher.

Yes because there is no point in finishing if you don't have the surprise.

Depends but for the most part it does for me. Like one user said the suspense is ruined so then I become less invested in the details between point a and b.

If I'm into the story, yes. Feel like this kind of question doesn't understand that a lot of stories only have going for them resolving one or two questions. Once you know the answer the rest of the story is so average it's hard to care to finish. For example I read a lot of otome isekai (tl;dr romance novels where the heroine was sucked into a novel/game). Once you know the twist it's just a subpar story.

That said if a story is kind of shitty and I'm considering dropping it, but I read a spoiler the twist is actually really good and puts the crappy parts in an interesting context I'll keep reading. So case by case, reader by reader basis. I have stories I only finished because I read a spoiler, but I have just as many I never finished because I read a spoiler.

For me specifically, if spoilers hurt a book, it probably wasn't worth reading in the first place. I love when authors demonstrate mastery of language and narration, and no amount of spoilers can overshadow the direct experience of witnessing it enacted.

Yes, it does. It is part of the fun.

Not really. Dramatic irony exists for a reason.

https://www.britannica.com/art/dramatic-irony

So, for example, Titanic. We all KNOW what's going to happen. The question then becomes how the characters deal with it.

Yes

It depends how important suspense/mystery is to the story, and if you enjoy suspense/mystery.

If you enjoy suspense, and a story is elevated by suspense, then spoilers matter.
If you don't care about suspense, then spoilers don't matter, and you don't appreciate when a story makes use of suspense (nothing wrong with this, people can like different things).