I have, in the past 10-15 years, become more interested in what the shadows hold than what the light is. Granted, both have to be good before I'll press the shutter button, but for years I was mostly trying to work in the area of the highlights and bright areas. All that does, though, is focus the viewer's attention on what was already the most obvious part of the scene.
We are, I think, evolutionarily afraid of shadows, of the dark. Lurking in that dark cave or moonless night could be terrible dangers, even death. It probably didn't take long for early humans to realize that some animals can see us at night, and therefore they can eat us. So we tend to avoid paying too much attention to those places where the light doesn't fully penetrate. My thinking is that what is only suggested, hinted at, in the dark places are the places that give structure and meaning to the scene.
Consider the Rembrandt painting below. "The Denial of Saint Peter" is at the Rijksmuseum in Amsterdam. Depicted is Peter being accused by the woman next to him as being one of Jesus' followers, which he denies with a dismissive gesture of his left hand. Two roman soldiers hear her story and weigh the situation, one of them seated at a table.
Eye-tracking studies of individuals viewing still images have shown that people first go to the brightest spot in an image, and then look for sharpness and contrast, and the last step is to take in the content of the image. In this painting, the brightest area of the image is from a burning candle just behind the maidservant's hands. The bright folds in Peter's robes and the white fabric in her dress immediately draw the eye to the center of the composition. And though Peter's face is slightly brighter than her face, the lighting on Peter is softer, seemingly more diffused. The light defining her features is crisp and contrasty, perhaps to highlight her youth. Peter's hand begins to fall in to the shadows, which I think suggestive of the idea that his denial won't stand up to the light of truth.
But there is something truly dramatic about this painting that could easily be overlooked if we didn't pay attention to the shadows. In the upper right of the painting, Jesus is being led away in chains to prison, and he is looking over his shoulder back at Peter. For me this gives the crucial bit of context for the scene. Once aware of that bit of information, I find myself wanting to study the dark areas of the painting more closely. What can I glean from the stances and expressions of the several people depicted in that corner? Only what one can imagine. Even Jesus' face is too obscured to clearly show any strong ideas. So we fill in those gaps and imagine perhaps his face is harsh and disapproving, while on another viewing we might find it soft and forgiving.
This bit of forcing us to imagine the larger, perhaps more chaotic scene is what brings the image alive for the viewer. We are forced to engage with the image, not just be passive spectators. The bright areas are obvious, and give us a clue as to the meaning of the scene, but it is the darkness that reveals the truth.