Millennial here (aged 32). I don’t use subtitles with everything I watch, and when I do use them it has nothing to do with keeping me focused. Although I can understand how it would help with focus if you’re someone with ADHD.
For me, it’s mostly about making sure I don’t miss a line of dialogue. It’s a common problem in movies, whose audio tracks are mixed for the theater and not for the home environment. I’ll get loud explosions where I have to constantly crank the volume down to 20% so as not to wake up everyone else in the house, and in the next scene the characters are whispering so that I’ll have to crank the volume back up to 80%.
Okay, maybe I’m exaggerating. Usually, it’s not that bad. But it does seem like the sound editing has gotten lazy with movies these days. It’s very frequent now that I’ll find a scene where background music or sounds will cover a word or two of dialogue.
When I watch films with older people (usually my parents), they have an even harder time catching every piece of dialogue than I do — the difference is that they just don’t care. They’re much more willing to let the scene keep playing and figure out what was said from context. Or to just not figure it out — with my parents, I’m constantly having to pause the movie and explain plot points, or who certain characters are. Which is not to say that all people their age are like that, but…
Anyway. Back to the subject of focus. Another benefit of subtitles, I’ve found, is that when a movie throws a lot of dialogue or plot at you, it’s easier for me to weave through the plot threads and keep track of everything if I have subtitles on.
I suppose my point is that it allows me to immerse myself deeper into the story, if I’m both reading and hearing the dialogue.
Watchmen is my go-to example. I liked that movie a lot more when I watched it for the second time, with subtitles enabled.