>There's no benefit to it only being able to reach some small group of people.
If something can just be locked to only talk inside the intranet/VPN it's better from something that can talk to arbitrary people the world over and is only configured not to via its own groups and permissions inside.
I thought we were talking about Slack, and several other hosted services, which don't fit the description you've just given, at all. But, for a self-hosted chat service, then yes...that's a benefit.
If something can just be locked to only talk inside the intranet/VPN it's better from something that can talk to arbitrary people the world over and is only configured not to via its own groups and permissions inside.