I'd be interested to know how it could be a good thing.

I'm all for shutting down Facebook entirely and jailing their executives, but i don't think age verification is an appropriate answer. It's already common at least here in France for Facebook to request user ID cards (wtf) and that has not at all stopped harassment, nazism, fake news, etc...

You're right, some people are misguided and think age verification can be done in a way that isn't the death of privacy and anonymity online.

Some people think it'd be reasonable to have every car reporting it's GPS position and speed to the government to stop people speeding, or to have facial recognition cameras on ever corner to catch criminals, or to make everyone carry an ID card that people can demand to see.

On the face of it living in a police state is safe, and secure, and actually increases your freedom to just live your life without pain. And if these systems were never abused that might be true. But sadly, the reality is that every time these things are implemented they end up being used against the citizens. So they're always a terrible idea.