Had to do some research to understand what they do. Here's what I found on the internet.

ALPR cameras scan thousands of license plates in the same amount of time it takes an actual person to scan a hundred. When a license plate image is compared against a hotlist or whitelist and matched, an alert is signaled and the screen shows the vehicle and the reason for the alert.

How it works. As a vehicle passes an ANPR camera, its registration number is read and instantly checked against database records of vehicles of interest. Police officers can intercept and stop a vehicle, check it for evidence and, where necessary, make arrests.

The newest technology in ALPR has added a new, helpful tool for law enforcement to catch criminals. There are also warnings that the ALPR systems can be misused and abused for inappropriate reasons.

Although ALPR data has been used to determine whether vehicles were at the scene of a crime, for example, or to track people down to fine them for minor traffic violations, that data can also be used to surveil and even target drivers who attend political events, go to gun shows, visit mosques, or drive or park near other sensitive places – whether or not the people with these specific license plate numbers have ever been accused of a crime.

Below is a link to a good article about the readers and details on how they work. 

Good Article On Licenses Plate Readers