Most guys that sand paint have one or two of these pads lying around.
Sanding pads help to evenly distribute applied pressure while keeping the sandpaper flush to the surface being sanded. This particular sanding pad is soft on one side and firm on the other, so it is versatile to use.
Another nice thing about these pads is that they are easily cut with a sharp knife or razor blade, so you can customize their size and shape for various jobs.
They're handy, so I keep one in my automotive sanding kit and another in my woodworking tool box.