Ziggurats were built in Urruk, and by 4000 B.C. they were being made on mud-brick platforms. They were on these platforms to make them seem larger, like a mountain. The whole ziggurat was made of this same material, and it took many, many men to build these things. It took years to finish one. The ziggurats in the city represented how great their gods were. However large the ziggurat may be showed how long that certain god/goddess had been worshipped in the city. (They were dedicated to the most important gods/goddesses.)
Human need, interest and curiosity caused the Mesopotamians to built ziggurats. They were interested in and curious about their gods that they believed in, and they believed that they needed to build these things to please their gods. They invented ziggurats so that they could make their gods happy and they would give them all they needed to live.
The Mesopotamians building these ziggurats reminds me of us making buildings in the present day. We build them for things that we need, and they usually give us something in return. We probably got the idea of building large structures from other ancient civilizations, and we would never be where we are today if we would have never learned how to build.
