The answer to this question depends on the time period in American history you are talking about. If this question was posed about the Colonial Period then the answer would be for agricultural land. As settlers poured into the thirteen English colonies land eventually became scarce. Settlers began to seek out land west of the Appalachian Mountains which led to conflicts with Native Americans who had gone there to get away from the English. Additionally it led to conflicts with the French as they claimed that territory for themselves.
The Louisiana Purchase and subsequent explorations by Lewis and Clark further drew people to the seemingly limitless and virtually free land to those willing to take on the expense and risk of getting there and settling the land.
There were also on-agricultural pull factors that led to westward expansion. Technology and jobs drew people west. The canal, railroad system, and the Trans-Continental railroad required an incredible about of labor, and many of those workers used their earnings to make a new life in the west. Additionally the discovery of gold in California in the 1840's drew hundreds of thousands of people west in search of riches. While most didn't find the fortune they sought, they almost always stayed resulting in a population boom in California.