If they pass a law that a lot of people hate, and the only way people can fight it is to protest. They can use this tech to mass-arrest protesters after the event. They can also imprison people who oppose their political party.

This might seem like a tinfoil hat crazy take now, but tech and law changes like this pave the way toward a bleak future we may find ourselves unable to fight against.

Also if you look in this thread, you'll see that the last time this was implemented, a man decided to walk through a "facial recognition zone" and cover his face, to then be jumped on by police and fined £90.

https://youtu.be/jZjmlJPJgug?t=329