I'm not sure what their business model is, but having such a large customer base is pretty huge in it's own right. I would imagine that they branch off to offer some sort of "premium" offering with some extra bells and whistles, given the right price point and the fact that the base platform is free, I think they'd make a fair deal of money.
It makes perfect sense to use something like Discord or Curse for VOIP, it's managed for you and has no real drawbacks to other private, paid services