Obviously needed in that scenario. That doesn't explain my neighbor, the Marvel Universe fan, but maybe he's an anomaly. I just had the sense there were a bunch of people out there like him who watched movies with subtitles as a matter of course.
I always have subtitles on, It helps me focus/some movies like garbling their audio a bit too much for my taste
I use subtitles in everything I watch, especially new movies, because they seem to lower the dialogue so much and explosions are so loud. It helps me keep the volume at a decent overall level and I won't miss anything.
I don’t need them for old films (pre 1990), but it makes it so much easier to tell what’s being said in newer ones. Clear enunciation gave way to stylistic accents after that - also, the sound is mixed with way too much range (works okay if you have cinema grade sound).
I've heard from some autistic people that it helps them in some way when watching.