So that the video is accessible to people who are hard of hearing?

ha like this guy https://www.youtube.com/watch?v=CN666q3ptAU

Subtitles is good so that you can speed up the video beyond the point where you can no longer hear what someone is saying. You can also watch without sound.

For the hard of hearing, non-native speakers who are used to subtitles from non-dubbed movies, etc. etc. I suppose.

I do know that, as a non-native English speaker, I've always appreciated having English subtitles for English-speaking films. It helps immensely if you miss a word, there's a little too much mumbling, or whatever. One of the best (in both ways) examples being the movie Snatch.

There's a curious habit in US media to put subtitles on anyone with an accent.

I would like to think it's to make video easier to watch without sound, or for the deaf, but I don't think that's the reason.

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