Yam was an efficient cash crop,it's tuber can be used as food and medicine.

Chinese Yam also be called as Chinese yam are cinnamon vine, and shan yao.Chinese yam is used in Chinese herbal medicine, traditionally to treat disorders related to the stomach, spleen, lungs, and kidneys. 
The roots of Chinese yam contain diosgenin, which can be used to produce steroids such as estrogen and progesterone in a lab.
While the tuber and bulbs of Chinese yam are edible, generally only the tuber is consumed as food.