Answer:
To protect migrating Native American tribes, and the US border with other countries
Step-by-step explanation:
These and other forts were built in the early 1800. At one hand they should be used to monitor and protect the US borders in the west, from territories in the south, occupied by Spanish, Mexican and French (Louisiana, Texas, for example. They weren't part of the US then).
By the other hand, with the Indian Removal Act in 1830, some Cherokee tribes were relocated in some west territories, at the other side of the Mississippi river. This, in exchange of their ancestral lands, now occupied by white people. Conflicts with Osage people in these territories existed and maintain peace in those new colonized lands was necessary.