Would be very handy for Accessibility reasons to support subtitle files for videos (our museum creates them for videos we produce). Instead of having to have the video editors include it with the video (faster to do fixes with a quick srt file replacement instead of having to re-export the full video)
We can already do ASL signers as video with alpha channel as layer over the main video.
And with the newest audio features, being able to setup a Descriptive Audio track in sync with the video as a multi-channel audio output increases the
I'm sure there are some other cases this would be useful as well for groups needing to do text overlays