What do you do in the following awkward situation? your friend receives an invitation to a party. The host is also your friend but you haven’t received an invitation.
Was the invitation lost in the mail or were you not invited? You can’t ask the host directly because it would be too uncomfortable if the answer was you weren’t invited. But in the event that the invitation was lost in the mail it is in all parties’ interest in having that uncertainty resolved.
There would seem no custom that would allow communication of the good news and at the same time avoid communication of the bad news.
But RSVP does exactly that, as long as the custom is to RSVP both acceptances and regrets. Then if you were invited but you do not RSVP the host will know you didn’t get the invitation, and send a followup.
Game theorists will notice that the bad news can still be inferred. If the host does not follow up then you learn that you were not invited. But the beauty if this system is that it is never common knowledge. The host never knows with certainty that you know about the party you weren’t invited to. You know about the party but you know that the host does not know that you know, etc… This higher-order uncertainty goes a long way in alleviating the awkwardness.
More generally there is value in social conventions that allow non-public communication: exchange of information, especially bad news, without making that information common knowledge.