The power needed to overcome the evil will yield a power that can reach the heights of holiness. A field has to be treated with manure and the manure has to ferment in the soil; similarly, humanity needs the manure of evil in order to attain to the highest holiness. And herein lies the mission of evil. A man’s muscles get strong by use; and equally, if good is to rise to the heights of holiness, it must first overcome the evil which opposes it. The task of evil is to promote the ascent of man.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science – Lecture VIII: Good and Evil – Stuttgart, 29th August 1906