Based on Chapter 4, what is the fundamental purpose of a saint's incarnation, and what happens when their mission is complete?

The fundamental purpose of a saint's incarnation, as explained in Chapter 4 of the Sai Satcharitra, is to counteract the decay of religion and uplift humanity. Saints descend to protect the system of castes and stages of life, destroy irreligion, and protect the humble, poor, and weak. They are described as being naturally liberated, incarnating purely for the benefit of others with no self-interest. The chapter concludes this point by stating that they work to awaken faith and establish the Ultimate Truth. Once their specific, divinely ordained work is fulfilled, they end their incarnation, their mission on Earth being complete.