Face plugs are simply shims with a very high friction co-efficient. They provide grip as well as lifting your workpiece away from the face of the clamp. Grip means that a cut on a CNC router will have less movement during a cut, which gives a better edge finish. By lifting the workpiece away from the face of the clamp a full depth cut can be made, and not damage the clamp.
The slideshow below will provide an understanding of how to use the faceplugs in an actual operation