Why do these holy Manifestations of God appear?

Why do these holy Manifestations of God appear? What is the wisdom and purpose of Their coming?
What is the outcome of Their mission?

The holy Manifestations of God come into the world to dispel the darkness of the animal, or physical, nature of man, to purify him from his imperfections in order that his heavenly and spiritual nature may become quickened, his divine qualities awakened, his perfections visible, his potential powers revealed and all the virtues of the world of humanity latent within him may come to life.
These holy Manifestations of God are the Educators and Trainers of the world of existence, the Teachers of the world of humanity.
They liberate man from the darkness of the world of nature, deliver him from despair, error, ignorance, imperfections and all evil qualities.
They clothe him in the garment of perfections and exalted virtues...

`Abdu'l-Bahá, The Promulgation of Universal Peace P.466