When Hinduism degenerated on account of creeping in of superstitious beliefs and false worship, various reformers appeared to purge Hinduism of its superstitions and wrong beliefs and observances.
Sri Sankara appeared to remove wrong beliefs and abuses in Buddhism and Jainism and established Kevala Advaita or absolute monism.
Sri Ramanuja appeared to teach Bhakti and make people practice devotion and realize God while remaining in the world.
Similarly many Acharyas appeared in this great land of ours. If the people became cruel, they appeared to preach Ahimsa or non-injury. If they become timid, another comes to infuse courage in them. If they become inclined towards rigorous Tapas (austerities), another comes and preaches "Realize while enjoying in the world".
Here is a compilation of what Gurudev Swami Sivanandaji Maharaj wrote about the great Acharyas who were born in Bharatavarsha (India).