When designing PCBs that will be produced professionally (or even if you etch yourself) it is sometimes hard to get a good view of what they will look like.
Most CAD programs view the design in a way that is useful during design but that is not what they will look like in the end. Some provide a view mode or 3D rendering option to get a good idea, but at least CadSoft Eagle does not.
What if you want to view what your design will look like or if you just want a fancy graphic to show off your wonderful PCB? Photoshop comes to the rescue.