By duplicating the Background layer you save a kind of a backup copy of your original image. Also, it enables you to correct the effect of sharpening, retouching, painting, etc. even after you re-open the image.

Note that in Photoshop 7, in case you duplicate the Background layer by using [Ctrl+J] shortcut (command Layer Via Copy), the copied layer inherits the locked position property (animation).

For more details on layers read the Skills chapter.