"The Significance of the Frontier in American History" and subsequent essays and books by Frederick Jackson Turner changed the way historians thought about the United States by arguing that it was the frontier that shaped American history.
First, the frontier made land available to people living in the increasingly expensive and crowded industrial east, defusing the sort of tensions about land ownership the caused civil unrest in other nations; those who were without land but wanted it could acquire land by homesteading in the west.
Next, rather than immigration causing overcrowding, immigrants could move west; many western settlements were created by immigrant families. This reduced potential conflicts between immigrants and residents over property.
Finally, as the federal government controlled and distributed the new lands of the west, western frontier lands provided a way for the government to give economic benefits without raising taxes.