Why do they need to scan license plates? They already have a complete list of plates, names, and addresses at the DMV.
To know where the cars are.
The official story is that they're looking for reported stolen cars or cars with stolen plates (although I can't see that given how fast they go through the lots, as they'd have to look at the vehicle associated with that plate), cars belonging to people who are being sought for some reason (warrant, felon, etc...), but the reality is the data is being collected on all plates. So, they can get an idea of who is where.
“Why do they need to scan license plates? They already have a complete list of plates, names, and addresses at the DMV.”
I suspect that the pigs have a bounty-hunter type contract with a private company. When the company finds a hot car (i.e., a car with unpaid parking tickets), they get a reward, as the car is disabled or towed.