Some cops have a system in place that automatically runs license plates when they're behind a vehicle so without even looking at the license plates the police can scan the license plate and be alerted to whether the vehicle has current license plates or not.
Then if you do not have a current license plate that is expired the police can then pull you over and give you a ticket for having an expired license plate.
Cops will sometimes run your license plates when behind you but not always if they don't have the automatic license plate scanner.
If a cop sees you violating some traffic law then the cop can run your license plate and see if the vehicle is current with registration or if the vehicle has been reported stolen or in some robbery or other crime.