Why is the Karnak Temple so important?

Second largest ancient religious site in the world

One of the oldest extant ancient monument sites open to the public

Because it was developed and added to over a long period of time,it is important as a source for studying ancient Egypt

For the Egyptians,its importance was propaganda for the Pharaohs.
Pharaohs were represented as living gods to the Egyptian people,and Karnak's temple complex was an important site in the national religion,so Pharaohs used it to boost their image and status.Many of the inscriptions describe the achievements and victories of Pharaohs,basically saying how great they were.

http://www.touregypt.net/karnak.htm