While fulfilling their own need to exist, develop, and reproduce, sweet potatoes also fulfill a purpose for the whole, in providing food for other beings. This is know as the principle of dual purposes, one of the fundamental principles governing the relationship between entitites.
As far as humans are concerned, sweet potatoes fulfill not only a physical or external need (or "hyung sang"), but also a spiritual or inner purpose ("sung sang"). Physically, they provide excellent nutrition for the body, and indeed, are one of the most nutritional vegetables available. Spiritually, they fulfill an aesthetic value, whether through the beauty of their flowers, or the joy of feeling the texture and seeing the colors in both their skin, which can range between red, purple, brown, and white, and their flesh, which ranges between white, yellow, orange, and purple. The taste, likewise, stimulates the taste buds, providing a joy beyond that of just the nutrition received.