In Europe they are meant to check the name on the boarding pass against an id such as a passport.
I believe that the reader just shows your name.
I also think what the readers do is update the system to say that you are on that flight. This is for revenue reasons. Tickets can be issued by other airlines, and the revenue is not counted until you are on the flight.
Also in Europe the system matches you with your luggage. This is a security measure.
It's also for manifest reasons. They need to have information on who and what is in the aircraft to calculate takeoff distance, power, flap settings, etc. and to account for everyone in the event of an accident or someone at CBP getting bored.
I believe that the reader just shows your name.
I also think what the readers do is update the system to say that you are on that flight. This is for revenue reasons. Tickets can be issued by other airlines, and the revenue is not counted until you are on the flight.
Also in Europe the system matches you with your luggage. This is a security measure.