Someone may have looked at a message but not replied. And so it wouldn't show up in "unread". So if my team is reading and cannot respond there and then, it's a nice way for us to see what is still unacknowledged (unresponded to). Only when someone has responded to a message would they mark it as acknowledged. That means we could then filter on "unacknowledged" and see all messages we are yet to respond to, including those that have been read or unread.