Traditional closed captions are still available, of course. Those are labeled “English CC” whereas this new option is simply labeled “English” (or whatever your preferred language is).
This is just the latest answer to a long-standing problem. Streamers and device makers have been including speech or dialogue boost audio modes to help users better understand dialogue, too. There’s no single reason this is now a problem that needs solving—rather, it’s a confluence of things.
The performance style of actors in current TV shows and movies is more naturalistic and less elocutive than it once was, so characters are more likely to speak softly. Streaming services compress the audio more vigorously than is common in physical media, which can cause problems with intelligibility. The range of home audio hardware is far wider than ever before, so engineers and editors mastering shows and films face the near-impossible task of providing strong dynamic range and quality for those with high-end systems, but still making it sound good on cheap TV speakers.
It’s a relatively new problem, and everyone from hardware makers to audio engineers is seeking the ideal solution. Netflix’s new subtitles don’t really fix it, of course, but they make the viewing experience a little nicer for the millions of people who’ve resorted to reading subtitles all the time.