What is the selfless mission of saints when they descend to Earth, as described in the fourth chapter?

As detailed in Chapter 4, the incarnation of a saint is purely for the sake of others, with no self-interest, as saints themselves are already liberated. Their mission is multifaceted: to protect the system of castes and stages of life, to destroy irreligion, and to protect the humble, poor, and weak. The chapter explains that they work to build the 'temple of Ultimate Truth' upon the foundation of Renunciation (Nivritti) and uplift devotees. Ultimately, their purpose is to perform the work of religion and awaken faith, and once this specific mission is fulfilled, they end their incarnation.