Clipping Mask uses: An Edge on your image
While there are many different ways to use clipping masks a very simple thing to do is to use them for putting various edges on the frame of your image. Auto FX has some awesome software that has thousands of edge masks and it is a great tool. If you dont have Auto FX this short little video will show you one simple way to make and use a clipping mask for various edge effects.
To view this video just click on the link below. Note: If does not show up underlined or blue in some web browsers but it is a live link.