The cardboard sail panels were actually there to prevent the headliner from billowing / puffing out when you closed either the doors or the trunk. Unless you have the windows open, the sudden rush of air from closing the door or trunk will cause the headliner to "inflate" like a balloon in that area since the area behind it is all open and would be the path of least ressitance. Having the cardboard in place acts as a deflector so it will not cause the headliner to tear or become unglued and cause it to sag in that area.