Basically, when police aren’t needed for something else, they drive around neighbourhoods and their equipment records the license plate of every vehicle they drive past, and records the location and time. This also occurs when they are to and from police business.
The massive database they create is useful for proving when and where a vehicle was as evidence for crimes. Of course that is good.
It is also surveillance of mostly innocent people, and such data could be used unlawfully.
It is the equivalent of phone-tapping everyone, except phone calls are “private” and your car being somewhere is “public”.
More here:
https://www.technocracy.news/police-use-license-plate-readers-to-grid-neighborhoods/