It's the legacy of the national surveying system begun by the Land Ordinance of 1785. [0]
Except for the thirteen original states, almost the entire country was surveyed and platted into square "townships" [1] of six miles a side. These were subdivided into square-mile "sections", and roads were built along section lines. Where there were no major rivers, towns were generally established at the intersections of these roads.
For more historical color (and stereotypes), try "Why the Midwest is Square" [2].
Except for the thirteen original states, almost the entire country was surveyed and platted into square "townships" [1] of six miles a side. These were subdivided into square-mile "sections", and roads were built along section lines. Where there were no major rivers, towns were generally established at the intersections of these roads.
For more historical color (and stereotypes), try "Why the Midwest is Square" [2].
0. http://en.wikipedia.org/wiki/Public_Land_Survey_System
1. http://en.wikipedia.org/wiki/Survey_township
2. http://www.surveyhistory.org/why_the_midwest_is_square1.htm