What is the purpose of gray, black and white?

The purpose of gray, black and white is to create infinite energy. When you combine black and white energy, the result is a perfectly balanced shade of gray.

Pure white and pure black are too extreme which makes it easy for them to intersect with their opposite extreme.

White can be seen as Extreme Spiritual which intersects with Extreme Physical (black).

Black is Extreme Physical that intersects with Extreme Spiritual (white). 

Through this constant movement, energies are shared between black and white. The act of intersecting dilutes both extreme energies forming a balanced shade of gray.

The illustration below shows how black and white intersect with each other but can reach a balance within gray.