I've been to several meetups here where the company has larger offices in a high rise tower and e.g. the meetup is on the 4th floor but the door bell rings in the 3rd floor reception, and because it's after work, no one is there. People arrive in a 30min window, so someone would have to stand at the entrance, or put signs everywhere, or let people wait. As I said, situational, but "scan code to let someone's personal phone buzz and not have their personal phone number posted anywhere" works as one case.
The other one was a party at someone's apartment and I only had their IRC and twitter handle but not their phone number, and they had a key pad at the entrance (long story, not their fault) and I didn't have the number. And yes, I tried to ping them but they seemed to ignore their phone so I had to wait for other guests to arrive anyway.
My third example would be temporary event locations where there simply is no door bell or the organizers are not where it would ring and again, a phone works but it would leak the caller's number and the organizer's number whereas I suppose you can just uninstall the app or have different URLs generated that stop buzzing after a while.
