The chinese to the north had better farmlad and the barbarians the chinese called them to the south would invade them and try to get food etc... so they built a temporary wall to keep out raiders, and have the chance to escape or prepare for battle. But by 600 b.c. china was divided into small states and were fighting to control all of china. Zheng ended up wining. Aka Qin Shi Huangdi decided to build a wall to protect himelf and all of china from invaders but not just a wall a Great Wall.

They built it to protect China from enemies

I hope you do know that it has the most population in the whole wide world