The westward expansion era in the United States was one of great importance, as it helped to create the shape of the United States today. One of the reasons acquiring land was important was because it would help to complete America's manifest destiny. Manifest destiny is a term that states that it was America's god- given right to control all land from the Atlantic to the Pacific Ocean. This belief that it was supposed to be America's land caused them to buy land (like the Louisiana Purchase from the French and Florida from Spain) and even fight for it (Mexican-American War- results in US getting Arizona, California, Utah, etc.).
Besides completing America's manifest destiny, acquiring this land allowed the US to gain significant natural resources. A perfect example would be in California. After gaining California from Mexico, gold was found. This lead to the California Gold Rush of 1849. Finding this valuable commodity lead to a huge increase in the population of California and it gave the American economy a boost.