It could be useful if someone is not a native english speaker because 4k blurays have only english language. So if you have a big 4k bluray file and a small one with your language you can play the audio track from that file. Or you can add additional tracks on top of the original, like commentary etc…
You could simply mux the audio track of that 2nd file into your movie – this way users can already pick from the available tracks.
There’s already an existing feature suggestion asking to support “external audio tracks” (same as external audio tracks). From what I understand their target is to use this when storing their media on cloud storage in order to avoid syncing the full file back and forth. Even if you’re coming from a different use case, this will also work for what you’re asking for here.
I suggest you comment/vote in that thread in order to avoid distracting/dispersing votes.
Therefore I’ll close this suggestion as a duplicate.
2021 clean-up: duplicate
