For me, it’s several things:

As others have mentioned, sound mixing sucks on a lot of newer movies. Watching a movie with a pleasant max volume could render dialogue inaudible. Holy crap, that last batman movie was hard to watch.

While I watch movies, there are often a lot of things happening around me. The dog might be playing around, my girlfriend is having a phone conversation or there are noises from the kitchen. I am not great at mentally filtering out sounds, so subtitles help a lot.

I live in a country where subtitles have been around my whole life. Now that TV is on-demand rather than linear TV with hardcoded subs, most services enable subs by default, and it doesn’t really bother me.