The reason Ksitigarbha Bodhisattva chose to have his three incarnations reincarnate was to cultivate another complete Bodhisattva body.
Ksitigarbha intended to use this complete Bodhisattva body to grant liberation to all the wicked ghosts and fierce spirits within the Purgatory, thereby fulfilling the great vow he had once made.
Each of the three incarnations would undergo reincarnation, endure the bitterness of cultivation, and accrue merit once more. When their merit was perfected, the three would return and merge into one, achieving the status of a Bodhisattva. In other words, Ksitigarbha aimed to cultivate another self and then sacrifice this newly cultivated self to liberate the eighteen layers of Purgatory.
Chapter 696