What purposes did the giant earthworks of the Adena and Hopewell people serve?

"Earthworks were built for various purposes by heaping earth in piles and shaping them. Some of the earthworks were large burial mounds, others served as platforms for structures such as temples, and still others served as defensive walls. Mounds were usually cone-shaped, oval, or formed into the shape of an animal."

-World History, Ellis Esler.