Artworks that exhibit a full range of value are generally successful. It doesn't matter what type of art you're creating or what media you're using -- it's simply the case that, generally speaking, artwork that shows both light and dark value is more aesthetically pleasing to the eye.
Artists are able to create the illusion of light by being able to produce a wide variety of values. In our fifth episode on the Elements of Art, we explore how artists produce and use different color and tonal values.