Lewis and Clark were important because they undertook the first expedition to cross the western part of the United States all the way to the Pacific Ocean. During their expedition, they explored the newly acquired lands of the Louisiana Purchase, surveyed and mapped the land, established a U.S. presence for legal purposes, made friendly contact with many Native American tribes, and scientifically studied local flora and fauna.
Long before he became president, Thomas Jefferson had been interested in having an expedition cross the continent of North America. One of his goals was to find a water route to the West. The official reasons he gave Congress when requesting financial backing were to promote fur trapping and trade with Native Americans. He also wanted to promote settlement in the West by providing more knowledge and better mapping of the area. The expedition had already been planned by the time of the Louisiana Purchase, but surveying the new territory gave the expedition added usefulness.
Although findings of the expedition were essential in the exploration and settling of the West, by the nineteenth century it was all but forgotten and received scant attention in history books. Only in the twentieth century was interest in the Lewis and Clark expedition revived through the Louisiana Purchase Exposition in Missouri, the Lewis and Clark Centennial Exposition in Oregon, and the Lewis and Clark Bicentennial in 2004.