To live away from where they worked

Americans bought cars because they had bought into the myth of suburbanization. 

The suburb was really about the increase in land prices as the result of cheap transportation from the out-lying farmland to the city center. 

Henry Ford's innovation made it possible to build a car inexpensive enough for every workman to afford one. The assembly line coupled to the end-to-end integration of the production process (so that the raw materials could be extracted as cheaply as possible) drove the cost of production down to where the mass consumer could afford a car but it was still important for the mass consumer to believe he needed a car. 

For that to happen Americans had to come to believe it was important for them to live away from where they worked. At first the railroad barons developed the suburbs but then the car came along and democratized suburban sprawl so that the land speculation in the suburbs and the bulding of the roads from the suburbs to the cities became something any local banker or developer could undertake, with the cooperation of the leadership of the newly incorporated suburban municipalities.