It adds volume to the painting and makes it look 3D, without it the painting would look flat and cartoony.
Explanation:
The Terracotta Army refers to the thousands of life-size clay models of soldiers, horses, and chariots which were deposited around the grand mausoleum of <em><u>Shi Huangdi</u></em>, <u><em>first emperor of China and founder of the Qin dynasty,</em></u> located near Lishan in Shaanxi Province, central China
What had inspired him to paint his ground breaking painting was from when he had saw the African and Iberian art