Final answer:
The Lewis and Clark expedition was important because it provided detailed maps, established U.S. territorial claims, and gathered extensive scientific data, which aided future westward expansion.
Step-by-step explanation:
The Lewis and Clark expedition is considered a significant event because it expanded geographic and scientific knowledge of the then largely uncharted American West. Commissioned by Thomas Jefferson following the Louisiana Purchase, the expedition, lasting from 1804 to 1806, sought to find a river route to the Pacific and to evaluate the commercial potential of the territory. Though they failed to find a Northwest Passage, their journey was crucial for mapping the region, establishing relationships with Native American tribes, and opening the way for future traders and settlers.
Despite initial lack of recognition, the journey of Lewis and Clark allowed the United States to solidify its claims to the new territory and brought back valuable data on geography, flora and fauna, which laid the foundation for westward expansion. This achievement made an indelible impact on the nation's history, echoing Jefferson's vision for the expansion and development of the United States.