In a nutshell, new markets meant more money. Because of industrialization, America was producing more than it could consume. Rather than allowing the excess to become waste, it is a wiser investment to expand your market. Mass production is far more profitable as the expense of making the product is less.
In addition to selling our goods to others, we also needed their resources. Dominating an area where there is an abundance of natural resources, allows our companies to produce items at an even lower price.
Prior to the 1900s, Americans would not have been able to produce enough to satisfy the needs at home and abroad, and the transportation of goods/resources would have been slowed down immensly before the creation of the Panama Canal.