Papa Lo says the purpose of cities is to further our evolution. Cities led to settlements, allowing people to spend less time migrating to follow the herds and more time harvesting the plants they depended on to survive. Settling down brought the development of technology, which improved the amount of food a settlement could produce. With more food, there was more time to be spent on things other than hunting and gathering. That led to learning and the development of more technology, bigger populations, and bigger cities.