Face recognition. In terms of govt scale intelligence, and increasingly just plain OSINT. Not everyone is as tight with their pictures as you are, they probably auto upload them on Google Photos or Dropbox or something. You are now irreversibly linked to everyone else in that photo who probably has social media profiles, public contact emails, show up on people search websites etc. I don't want to cause problems for you just because someone wants to blackmail me. I also don't want you to tell people where I live because someone broke into your house and is removing your toenails with a pair of pliers.

Subjective opinion time, I just think it's lame. I don't sit back and reminisce over pictures. I don't want to be in your group picture. I want to hang out with the people in the group and have a laugh.