This tutorial can be used for at least three things:
to add texture to a picture to camouflage or disguise that the picture has unwanted texture
to add a subtle texture to any picture
to add grain to skin  [details (http://www.retouchpro.com/tutorials/index.php?m=show&id=14)]

Tell me, please, witch percent is Scaling:
Relief:
Light:

Unfortunately, this tutorial was written by Mig, who hasn't been around for awhile. But I suspect the settings you're looking for vary widely depending on the actual image.