When the United States became its own country in the late 1700's, most of the population was engaged in agriculture and lived on the East Coast. There was a minimal industrial base, as Great Britain had actually attempted to restrict the Colonies trade and industrial pursuits, preferring to keep them as a source of raw materials and to keep them from competing with the Mother Country. However, after the American Revolution, no such restrictions existed, and trade and industry flourished. Simultaneously, immigration increased, and new farmers necessarily had to move westward to farm. With the full development of a railway transport system, agriculture and industry intersected -- the railroad moved agricultural products Eastward to the cities, and moved manufactured goods Westward to the farms. This internal trade removed the necessity for most of the population to be engaged in agriculture, resulting in most working in factories in cities by the late 1800's, which had also migrated westward after the settlers and farmers.
Westward expansion was especially important in the Reconstruction Era that followed the Civil War. Additionally, it contributed to the conflagration of the many issues that ignited the Civil War; as the United States gained more territory, the question of whether the new states would be freesoil or slave-holding states was hotly debated.
After the Civil War, the West offered inspiration to citizens hopeful of taking part in and expanding the ideals of a free labor society. As the Republican party and special interests groups desired a more active government, those who were opposed saw the West as a place untouched by government authority. It was America in it's most raw, idealic form.