Stop in for a cup of coffee
It's just a visual aid to help you know when to reverse the turn as you are backing up...
It doesn't actually do anything but sit on the dash and the pointer is turned to an angle after you make a few passes at parallel parking. Basically it's designed so that once you get the angle set, you turn the wheel as you are backing up until the pointer is parallel with the street and then turn the wheel the other way to finish the maneuver.
It's just a visual aid to help you know when to reverse the turn as you are backing up...