Why was setting the nations border important economically to the new country?

During the 19th century, especially prior to the outbreak of the American Civil War, settling the western territories of the United States was economically important for several reasons. Intrinsically, having settlers in these lands would increase production and general prosperity for the nation as a whole. At the same time, the presence or absence of slavery in these areas would also have a national economic impact -- and a political impact -- by contributing to the overall national balance of pro-slavery and anti-slavery power.