Proper light and shadow effects in your graphics give them a real-world, 3-dimensional feel. Light and shadow effects also invoke different feelings of peace, melancholy, silence, professionalism and mystery. They can also give you a feeling of cold and warmth.
While creating light and shadow effects in your graphic presentations, you need to be very careful about the source of the light. There should be a perfect symphony between light and its subsequent shadow.
This blog post showcases some brilliant examples of light and shadow in web designs.