Trying to be as least cynical as possible, imagine if you were a party with 3 or more people, and you really wanted a nice pic like that to commemorate the night, but never got to take one all together because someone left early, and you were in the bathroom when they announced their departure.

Yes, it's a major first-world problem to not get a photo, but to the right person, it could mean a lot to them to "fix" the photo they took with A and B to add C to their 'rightful place.'