The following excerpt was taken from “The Summits of God-Life: Samadhi and Siddhi” by Sri Chinmoy.
Whenever there is a necessity, great spiritual Masters come down to help the yet-unrealised souls. They are playing their part in God’s Game, His divine Lila. Seekers get a tremendous opportunity when they are helped in their aspiration by real spiritual Masters, for by personal effort, without the help of a Master, it is very, very difficult to get realisation. Aspiration is of paramount importance, but mere aspiration cannot give realisation. There is a necessity for the divine Grace. And this Grace comes most easily through the intervention of Avatars, Yogis, saints and sages. The world is progressing and evolving as quickly as it is just because great spiritual Masters are coming into the world to help.
After working on earth for a few incarnations, the Master may want to continue his work for humanity from the other world. At that time, he knows all the hidden corners of the Mother Earth; he knows every heartbeat of the earth-consciousness, so he is able to work from outside.