Mostly the consistency, we play other games too with varying quality (or nonexistent) in game voice, so it’s convenient to just have a server that we connect to and use regardless of what game we’re playing.

For an Overwatch specific if more unusual use case, if you want to have a group of 12 people organizing private 6v6 matches, this is much easier to coordinate when you can all hop into a single shared voice channel between matches to chat, talk trash, reshuffle teams, etc.

I haven't used the Overwatch/Blizzard voice chat but can people outside of the game talk to people playing?