The reason Ksitigarbha Bodhisattva sought to have his three avatar manifestations reincarnated was to cultivate yet another complete body of a Bodhisattva.
With this complete Bodhisattva body, Ksitigarbha intended to deliver all the wicked ghosts and malevolent souls trapped within the purgatories to ultimate liberation.
Thus, he would fulfill the grand and supreme vow he had made long ago.
Chapter 696