This Sun and Moon was a precious treasure of the Song Clan.
Every great Sect and Clan possessed some valuable treasure. Only in this way could their continued status and glory be maintained.
For example, the Black Sieve Sect had their incense burner, and the Solitary Sword Sect its enormous sword. They were all precious treasures. As for the Song Clan, the sun and moon which hung in their sky enveloped the entire Clan, transforming it into a different world than what existed outside.