Why did sumerians build ziggurats

Ziggurats were temples. 
Like many ancient people, the ancient Sumerians believed that powerful gods lived in the sky. They built huge structures, called ziggurats, with steps climbing up to the top. Religious ceremonies were held at the very top. People left offerings of food and wine. The priests enjoyed these offerings, as the gods could not eat for themselves.
The Ziggurat was built in the center of town. It was the center of daily life. Except for festivals, which, for the most part, were gloomy things, the Ziggurat courtyard was gay and lively. You might see an artist painting, a boy racing by on his way to school, someone milking a cow or making a basket. 
From the top of the Ziggurat, you could see the protective wall built about the entire town, and over the wall to the farmlands beyond.