Westward migration was extremely important for social and political developments. Let us look at two ways in which this is true.
First, in terms of social developments, westward migration is often credited with building the American character. This idea is most closely connected with a historian named Frederick Jackson Turner. Turner argued that westward expansion and the existence of the frontier helped make Americans more self-reliant and more democratic than they would otherwise have been. It made them rely on themselves instead of on the government or social institutions. It made them more individualistic and democratic, as can be seen from the fact that western territories were the first to give women the right to vote.
Second, westward migration was extremely important in terms of political developments. The development of the political conflict over slavery is perhaps the most important example of this. As people moved westward, there came to be more conflict over the status of slavery in the new territories that they were inhabiting. It was this westward movement that led to the need for a transcontinental railroad which, in turn, led to the Kansas-Nebraska Act. This law helped to increase conflict and helped to bring about the Civil War.
In these ways and others, westward migration helped cause major developments in American politics and society.