Interoperability, zaps, and data sharing. If you could add someone by a known ID, have data cross the app barrier (like the in-game ID same as chat ID), maybe even do business (imagining gig-sharing, freelance co-op, or something of the sort).
Discussion
For private groups would you not perhaps have people who would not want a cross-postfoem Identity? In some instances at least? I suppose that could be catered for? No idea tho
Easy to make a new npub, but it would be terribly convenient to not have to if you don't want to. Kinda makes privacy opt-in, but it's not too difficult to run an anon acct. alongside a "normal" one.