I assume Paper Thickness raises the head accordingly. What does Platen Gap do ?
Paper thickness adjusts the transport mechanism to allow for the media, but doesn't change the distance from the head to the paper surface. This places the head at the optimum distance from the surface of the media. The platen gap is what changes the gap between the head and the paper. Widening the platen gap requires the droplets to travel further than they were intended and may reduce the quality of the image slightly, especially on high quality PK ink papers (such as EFP), but can help prevent head strikes.