Fortunately my "defenses" worked on this one, though CSS only doesn't seem to have been enough. I don't care if they want to make the user feel like they're living in a blingee gif by default, I just desperately want pages to respect prefers-reduced-motion

What are the times in which it is useful?

If those are referenced in the linked article, I'll be honest I didn't read it. That website succeeds whole handedly in its job of being too annoying to read.