To be successful as a designer it is necessary to be able to use value effectively. Value provides the maximum contrast available and is useful for controlling visibility and hence controlling attention.
Value as design element
Value refers to the relative lightness or darkness of a certain area. Value can be used for emphasis. Variations in value are used to create a focal point for the design of a picture. A light figure on a dark background is immediately recognized as the center of attention. This is also applicable for a dark figure on a mostly white background.
Value is an important tool for the designers and artists, in the way that it defines form and creates spatial illusions. Areas of light and dark can give a three-dimensional impression. This is similar to when shading areas of a person's face.
Dynamic range of Values
There are differences between how we see values in our experience and how the artist depicts those apparent values. In our world the lightest thing we are likely to experience is the sun, it is too bright to look at. The darkest would be a complete absence of light, like in a cave.
These extremes are much farther apart than the poles of value available to an artist -- white and black pigment.
To reconcile these differences requires that the designer either use one end of the value dynamic or compress the real world's value extremes into the tonal range between white and black pigment.
Using Values effectively
There are two major considerations when using value. First is the amount of contrast which controls visibility. Second is the tonal range of values used that controls mood and ambiance.
Using Value Contrast
The visibility, and hence the noticeability, of an item depends largely on how much contrast it makes with it's surroundings. Black and white are the extremes of value, the most different items can be from each other. When that extreme is used the contrast makes the item the most visible. When the contrast is lessened, the visibility is reduced so that minimum contrast produced minimum visibility
The range of values from black to white is the tonal range available to an artist. The ability to use that range effectively lets the designer determine the mood of an image and create and control ambiance.
Skillful use of Value as a design element allows a designer to control visibility and with that attention and emphasis.