How difficult it would be to communicate emotions verbally if our voices could rise and fall. The crash of a cymbal in a symphony adds excitement and increases our listening enjoyment. In each of these cases, contrast is enhancing the experience.
Contrast is an especially important principle in graphic design, it is crucial to communication.
Contrast is Automatic Putting a black shape on a blank page creates contrast. Divide a page in half, black and white, this is value contrast. Add a large element, and you have contrast of size. Strengthen an Idea Using contrast is often an unconscious decision. If you were designing a poster about the moon, you might use a dark background.