Answer:
So when people dont need to produce there own food they do other services and work. like making cloths and other things. but they needed to be within a close distance of whoever was making the food so they could trade there service. or product for the food. and there product for other products. so thats how cities came to be. and thats why populations are dense in area
in short TLDR : to trade stuff and work for the guy/girl growing the food