For one thing, many imperial policies of the United States have helped increase diversity at home. Puerto Rico and Hawaii became part of the United States at the end of the nineteenth century, and their residents became US citizens. A large Filipino population became firmly rooted in American soil at around the same time. People from abroad began settling in the United States even if they were not from American territories simply because of the prestige and wealth the country projected through its imperialistic efforts. This has helped create a rich diversity that defines US society today.
As far as economics is concerned, imperialist policies have led to huge debts for the country today. Imperialism led the country to strive for military hegemony around the world. To fund this massive fighting force, a significant portion of the federal budget has been devoted to the military. This has led to the country accruing huge debts from foreign creditors.
However, it isn't all negative. American imperialism has also led to a huge amount of international trade. Even though the United States never had the most overseas territory, that did not matter much for commerce. Imperial policies in the early 20th Century created huge markets for American goods that still exist today.