Chilling effect. If the government and corps know who you are, you're less likely to celebrate insurance CEO's getting retired or click like on leftist memes or even join an anti-capitalist meme group

Blackmail when that isn't enough. It presents another chilling effect due to knowledge of it. And if actually used it would likely be used to turn people into informants and fed actors within groups rather than actually dissuade massive groups of people, still a big problem.

Criminal prosecution. People are going to say and do things that through tortured interpretations of laws are legal violations, real threats, whatever, and at some point these might be used in semi-targeted but broad ways (say they have facial ID of you at an anti-ICE protest, they go and arrest you and everyone else and then use your internet history to pile on additional charges besides rioting or assaulting cops or whatever as well as to slander you in the media if you ever become a cause celebre).

Intelligence gathering and network mapping. NSA is good but use of VPNs and other technologies due frustrate their ability to in bulk collection add names to every user. With required IDs they can create network maps and graphs of groups of people with total accuracy and assurance they know who is in the group, who they're talking to, what other groups they have connections to out to 12 degrees, 20 degrees of separation. This will be critical for combating and controlling domestic insurgency if things continue to go the way they are.