What is the selfless mission of saints as described in Chapter 4, and how do they achieve their purpose?

According to Chapter 4, the incarnation of a saint is a completely selfless act, undertaken purely for the sake of others, as they themselves are already liberated. Their primary mission is to protect the system of castes and stages of life, destroy irreligion, and protect the humble, poor, and weak. The chapter beautifully describes their work using a metaphor: they 'fill the foundation of Renunciation (Nivritti) and build the temple of Ultimate Truth.' By performing the work of religion and awakening faith within the framework of worldly activity, they uplift devotees. Once this specific mission is fulfilled, they conclude their incarnation.