We can expend (or add a new plugin) to the multiplayer plugin by having Voice chat (or even webcam sharing) over the WebRTC we already have for text messages.
Nowadays, we communicate a lot more by vocal.
For some games, it help to coordinate more rapidly or because, the game is too busy / frantic, you don't have the time to write.
Yes, with friends, you could use Discord or something, but if you play with strangers by selecting a random / new server, you won't have an easy alternative.
Also, the best use case, is having special features integrated such as close proximity voice chat / global (like for moderators or game master) / teams.