From the WHC Inscription:
The combination of striking natural formations and the development of a complex system of canals, bringing the thermal water to nearby villages and fields, is exceptional. The springs are the source of a hydraulic system extending 70 km northwest to Alasehir and westwards along the valley of the Menderes River. Pamukkale forms an important backdrop to the original Greco-Roman town of Hierapolis and the cultural landscape which dominates the area.
Read More at: http://whc.unesco.org/en/list/485