It prevents the tv from receiving power when the ignition is on. This assures compliance with the federal law that prohibits having an active tv within view of the driver of a vehicle.
If you look at the back of the controller, you will see it has a wire that goes to an ignition-switched circuit. If +12v appears on that wire, it opens the 120vac supply circuit.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL