One of the important elements of creating a successful image is to have a variety of values. Here are two key aspects of value, one is that value is defined as the range of possible lightness or darkness in an image and two it is important to have contrasting values adjacent to each other for accenting objects. Just a note about contrast which is the result of comparing one thing to another and seeing the difference. Think about how two dimensional objects get their volume with the shading that uses a range of values.
Check out the same image below that is now just seen in black and white. Notice how important the contrasting values are in the success of this image. Without the element of color, the eye is only able to distinguish each item by the range of adjacent dark to light values. An example of this would be the dark value used in the back ground provides contrast for the light values on the container. Or the highlight on the round object is surrounded with darker values.