My brain is not wired to remember certain details about people. I can have a conversation with them for an hour, truly engage with them and pay attention to what they say because I care to do so, and a week later remember almost nothing about the talk.
I have a Notes.app record listing my coworkers and the important things I want to remember about them. I wish I could remember all these things without writing it down, but I can't, even when it's important to me and something I'm genuinely interested in. Something like Monica to keep track of all those social details in an organized, completely private and self-hosted way, would be a godsend for me.