Contrast adds interest to the page and provides a means of emphasizing what is important or directing the reader's eye. On a page without contrast, the reader doesn't know where to look first or what is important. Contrast makes a page more interesting so the reader is more apt to pay attention to what is on the page. Contrast aids in readability by making headlines and subheadings stand out. Contrast shows what is important by making smaller or lighter elements recede on the page to allow other elements to take center stage.
However, contrast can be overdone. Choose carefully. If everything contrasts highly with everything else you end up with competing elements and once again the reader won't know where to look first.