Why was the USA so intent on Immigration in the 19th Century?

There are a lot of Polish, German, Italien and Irish americans that emigrated to America in the 19th and 20th centuries, why did America promote emigration to their country and encourage Eastern Europeans and generally people who were poor to come into their country? Did they want a lot of people in their country because they were planning to be a superpower one day? or were they just doing it out of the goodness of their hearts?