Why was the Great Wall of China's commisioned?

The Wall of China was built as a protection.

Explanation:

The Chinese wall was built by millions of people in order to protect themselves from their enemies, people that came from the North, especially the Mongols.

Even though they built it up, the Mongols kept conquering China.

The role of the Chinese wall was also some kind of a border so the Chinese people stay within the country, it kept them from leaving China.