Why were cities essential in the growth of civilization

Cities resulted from farming techniques and an abandonment of nomadic tradition. Staying in one place required much foreknowledge - how to cultivate crops in one place, how to keep livestock from wandering, how to keep food safe during seasonal extremes. Because of this, they were usually based near a constant and reliable water source. In cities, people mingled with others, and trades began to develop as people were replaced with early tools and machines for farm work. This lack of need in the usual sect resulted in a cultural outpouring as people came up with creative ways to market their skills and knowledge to others. Hierarchies of these trades and positions began to emerge, and social structures began to evolve from primitive, simplistic responsibilities to more complex organization. This organization was headed by religious or government figures, who had the "most important" and therefore most respectable position. Without cities, this cultural outpouring, social hierarchy, and community organization would have never occured in nomadic tribes. 

Not sure if there was any sort of picture or something to go along with this but my guess is the book went into deep depth and was very informative.