Economically, westward expansion affected the United States mainly by opening up vast new amounts of natural resources. For example, westward expansion opened the Great Plains to farming. This allowed large amounts of grain to be grown on land that was really well-suited to the purpose, which meant that relatively poor lands in the East did not need to be used anymore. It gave the US access to sources of metals, including famous strikes of gold in California and silver in the Comstock Lode in Nevada. All of these resources and more were added to the US economy when the country expanded westward.
Politically, westward expansion exacerbated divisions between the North and the South. This was particularly true in the time after the Mexican-American War. When the US got the lands of the Mexican Cession, the North and South came into conflict over whether those lands would be free or slave. The arguments between advocates of “free soil” and those who wanted to spread slavery into the new lands made the North and South even angrier at one another. This helped to bring on the Civil War.
Socially, it is said that westward expansion helped to create the American character. We see ourselves as a land of rugged individualists, the heirs to the pioneers who “tamed” the West. Without westward expansion, we would not have our stories about cowboys and pioneers and other tough people who we see as archetypes of what we are as Americans.