It was the Mesopotamian version of a stepped pyramid.
It is like pyramids all over the world.
There is a simple explanation for the existence for pyramids worldwide, without delving into any of the various sorts of conspiracies.
Pyramids, for the longest time was the tallest buildings man could build with the materials on hand. It also limited how high we could because to build them higher would have required bigger bases and there is only so much cut stone that can be cut.
You see the same principle in cars. One of the big reasons why so many fuel efficient cars look alike is because there are scientific principles behind aerodynamics and fuel efficiency and if you want to take advantage of those principles, the vehicle has to have a certain shape to it. Any variation from that shape is going to hurt the fuel efficiency.