Why did Shi Huangdi have the Great Wall built?

Shi Huangdi had the Great Wall of China built to protect the Chinese states and borders against the raids and invasions of the various nomadic groups of the Eurasian Steppe. This included the Mongols to the north, which had been very powerful during the rule of Genghis Khan. The other purposes of the Great Wall were border controls, allowing duties on goods during the period of the Silk Road, regulation and encouragement of trade and the control of immigration and emigration.