Since Chichen Itza was a place where many people lived over the course of many centuries, it had a variety of purposes. But the answer you are probably looking for is that it was a ceremonial and religious center. This was the main reason for Chichen Itza's importance.
We can tell that Chichen Itza was a religious center because of the nature of many of the ruins and because of human remains that have been found there. For example, there are many examples of the remains of people who were clearly sacrificed to the gods in cenotes around Chichen Itza. As another example, there are important religious buildings such as Kukulcan's Pyramid. This large structure is believed to have been used for religious rites that were meant to ensure good agricultural results.
Chichen Itza's main purpose was to serve as a religious center for people in the region.