Wang Ping supported the Demon Race going out because he wanted them to share some of the pressure brought by the divine artifacts of West Continent. Moreover, the Demon Race truly possessed many good things that could promote the advancement of the cultivation world.
For instance, Artifact Cultivators might cooperate with the Demon Race in the future to raise their chances of advancing to the Third Realm. Their bloodline abilities could also be used to assist unorthodox cultivators in their cultivation, among other things.
Because of the great fire True Monarch Lieyang had set in Central Province, the unorthodox Dao lineages had nearly been wiped out. Without their support, relying solely on Taiyan Sect's strength to rebuild Central Province was nothing more than empty talk.
As for why Kai Yun had agreed to let the Demon Race go out, Wang Ping guessed that either he or Vajra Temple had set their sights on some of the Demon Race's secret arts. Previously, the Demon Race had remained huddled within the Demon Domain, and he could only follow tradition and fulfill his duties. But once the Demon Race went out, he could openly seek to obtain them.
Ao Hong might simply have found it interesting. Or perhaps he had indeed received certain promises from the Demon Race. Take the matter of driving away Taiyin Sect last time—without a joint decision by the Demon Race's Elder Council, Bailin would never have been sent to assist them.
Zhi Gong's thoughts were more or less the same as Wang Ping's, and she might also have received some additional promises from the Demon Race. She preferred for the Demon Race to go out, but wisely chose to remain hidden behind Wang Ping.
As for Prefect Yuxing, she probably truly just wanted to treat the Demon Race