It's also useful if you want to use different render settings on different areas of an image. For example you might find that certain (faster) settings look good enough for the whole image except one figure's hair which comes out blurry. You can do a second render of just the bit around that figure's head using the area render feature at much higher (slower)settings. Save both out and stick 'em together in P-shop, still quicker than waiting for a slow render of the whole scene.