So even though the common people suffer terribly, with tens of thousands dying from hunger, cold, or being captured and killed, the local strongmen and gate valves take advantage of this chaos to expand and grow, flying the banners of the Heavenly King, Immortal Palace, or Mystic Sect, and engaging in mass plundering of people, resources, and land.
And notice, it's plundering, not destruction. No one in their right mind would only focus on destruction—the ultimate goal of risking one's life is always wealth and glory. In the end, wealth and resources are merely transferred from one person to another.
So although the Heavenly King's rebel army takes the lion's share, most of what they seize are gold and silver treasures, money, grain, and women—resources that are actually not of much use to the Mystic Sect.
What does the Mystic Sect want?