From the WHC Inscription:
The Great Wall reflects collision and exchanges between agricultural civilizations and nomadic civilizations in ancient China. It provides significant physical evidence of the far-sighted political strategic thinking and mighty military and national defence forces of central empires in ancient China, and is an outstanding example of the superb military architecture, technology and art of ancient China. It embodies unparalleled significance as the national symbol for safeguarding the security of the country and its people.
Read more at: http://whc.unesco.org/en/list/438/