What conclusion have archaeologists and historians reached about the reason the Sumerians built ziggurats?

The reason the Sumerians built ziggurats is for the gods

To create a bridge between heaven and earth

Explanation:

The strange common temples in which Sumerians, Babylonians, and Assyrians gathered were erected, as Herodotus noted, so that at their top there would be a sanctuary. Still, no preserved sanctuary has been found to date.

These buildings also had a practical purpose - when the floods hit the Iranian Plateau, priests at their peaks could take refuge from the water. In addition, the sanctuary was not easy to reach because they were led by three rows of stairs guarded by guards to keep secrets of rituals performed at the top of the ziggurat.

Each ziggurat was part of a larger complex with a large number of bathrooms, living space, warehouses, courtyard, and around this complex a city was built and expanded. The Sumerians believed that the ziggurat was a kind of bridge between heaven and earth and that the gods, when residing on Earth, visited the temples at the tops of the ziggurats. For this reason, only royal servants and priests could enter these buildings.