King Tutankhamun is most important because of the quality and quantity of artifacts found within his tomb. King Tutankhamun's reign is not known as particularly important. The reign of King Tutankhamun centered mostly around reinstating the gods and building structures.
Due to his short reign and the change of religion his father had made, Tut spent much of his reign repairing the damage and restoring the old religion. He was also tainted by the memory of his father's changes. He was also sickly and died at 19, further limiting his chances of having a long, important reign.
However, the discovery of his tomb catapulted him into significance and importance in the modern day. Most of the tombs of pharaohs have been looted, so when King Tutankhamun's fully intact tomb was discovered, it was historically significant. His tomb was small, likely due to an unexpected death, but filled with thousands of treasures. The tomb included his intact burial mask, coffins and sarcophagus. All of his burial treasure was still intact, as well, which was unheard of for pharaoh tombs. His artifacts taught Egyptologists much about the tomb and burial traditions of the era, and his artifacts regularly appear in museums of the world. In recent years, the most delicate items have been kept within Egypt's borders for safe-keeping.