>>211437951 (OP)Fandom is a funny thing that people don't really understand unless they are really in one and a part of it themselves. Just being a big fan, even a really big fan, of something doesn't make you part of the fandom, for instance.
Being a part of a fandom means you form social connection with others based around your mutual love of a *thing*, or more specifically an IP of some description. It means they create a sometimes fairly deep emotional connection with the actors in the show/movie and experience genuine joy when they do a new interview, are seen saying anything publicly, or these days posting on social media.
Taking a photo with the actor who is from your show is a culmination of the parasocial relationship for these people. 'We have a photo together so we are mates now' or 'Hey, we both got a photo with x actor so WE have a connection'.
It's just a part of the parasocial world we live in, it's always been a thing, but it's more pronounced now.