I recently realized that I turn subtitles on almost everything that I’m watching and I think a lot of us do. Why are we all doing this now? One big reason is dynamic range - the range between your quietest and loudest sound. For movies, you want this range to be wide because if your dialogue is at the same volume as an explosion that immediately follows it, that explosion isn’t going to feel very big. It’s the contrast that gives our ears a sense of scale.
The thing is, there’s a limit to the direction that dynamic range can go in. You can only make something so loud before it gets distorted, so you have no other option but to push the quieter sounds quieter, instead of pushing the louder sounds louder. Explosions go up and dialogue comes down. Everything is trying to look and sound more cinematic these days and while that kind of mixing works really well, if you’re watching something in a movie theater, it becomes a lot harder to hear when you’re just sitting at home watching whatever, trying to ride that volume control so that we can both hear what the actors are saying without having our ears blown to bits by the sound effects. So you just put subtitles on.