Why is the louisiana purchase treaty an important document in the study of manifest destiny?

The Louisiana Purchase treaty is important document because it shows the extent to which the United States was claiming land westward.

The Manifest Destiny is a belief that the American expansions westward during the 19th century was justifiable and inevitable. So if you think of it that way, it illustrated the relationship between the United States and France  in a good way showing how it was justifiable and non-violent.