(a) Why was Karnak so important to the Ancient Egyptians?

Answer 1

It was important as it was home to the God Amun. The power of this religion gradually increased until it reached its apex with the Pharaoh Akhenaten. The temple complex was started during the reign Ramesses I. Worked continued during the reign of Seti I (1306 - 1290 BC). Seti's son Ramesses II continued with the construction. Ramesses II is considered the greatest of all the Pharaohs of the New Kingdom. It was also important because of its location at Thebes. Thebes was the capital of Egypt during the Middle Kingdom.

Answer 2

It may be tied to astronomy.

That's just speculation though.