Contrast is important because it provides visual hierarchy in a design. 

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Important elements of the design can be enhanced using contrast.  In other words, if your ‘SALE’ text is to be a primary element of your design, using contrast can help make that your focal point.  The easiest way to achieve this is by using color.  Color can create the barrier need to differentiate one level of the design to another. ("A Graphic Design Primer, Part II: The Principles of Design.")