Agriculture had an important part in Mesopotamia, the climate of the region was dry with little rainfall, which was a big problem. Once they developed the irrigation system that depended on the flooding of the rivers, they had food surplus. Food surplus means that the population has food to eat, to sell and also means that the population can grow.
They were able to trade their extra crops for other goods such as stone, wood, metal products, which made possible for them to built canals and ditches to bring water to the land.
As you can see, food surplus made possible for them to have a better and comfortable life, which also made possible for the population to grow.