A traveler in any country may well wonder why man builds cities in the places he does, when often there are environmental factors present which seem detrimental to human habitation—dry and infertile soil, isolation, excessive rain, or extremely cold weather. Cities are built to serve particular functions, which are usually of major economic importance and which are often independent of the environment.

Few if any cities have been built merely to provide shelter, and it is doubtful that any city with that as its sole function would survive, much less thrive. So-called bedroom communities are dependent on the core city they serve; and the new satellite cities must attract industry and commerce even to become populated. Besides providing a place to live, a city serves a regional activity (or a combination of activities) such as farming, manufacturing, finance, or government.

Because they must serve a region, cities are built in a location characterized by some geographical or geological advantage. Many cities of the eastern seaboard were built on the fall line, to take advantage of waterpower and water transportation. Some sites were chosen because they provided easy access to the hinterland and its resources. St. Louis is a prime example of a city that served as the gateway for the exploration of a vast territory. Though the location of a city may have been selected for reasons now irrelevant, it still exists in a biological and geological environment.

Cities have been located on the banks of rivers, at the confluence of rivers, around harbors, at the intersection of roads and/or railroads, near rich deposits of ores, near plentiful sources of waterpower, etc. in the Great Plains region of the United States (North Dakota particularly) the cities were distributed, like beads on a string, at intervals along the railroad right-of-way, each one serving a farm hinterland as a collection and shipping point. Larger cities have sprung up as the financial or cultural centers of vast regions. Minneapolis-St. Paul is an island of culture, education, manufacturing, and finance in a vast sea of agriculture. Similar centers are scattered through the central reaches of the continent: Milwaukee-Chicago to the east, Winnipeg to the north, Omaha to the south, and Denver to the west. Several hundred miles apart, they serve comparable hinterlands in a comparable way.