The polarizer just lets you adjust the reflections.
Because a car is an object with many surfaces all at different angles to the camera lens it cannot really get rid of all of them. That's why on the "with" image the reflections are minimized on the hood and windshield, but are strong on the side of the car and headlights.
Essentially you look through the viewfinder, adjust the polarizer through its full range and decide which effect you like the best.
I don't know if I explained that well.
