Why did the government wish to enter into treaties with the First Nations people?

The government wanted to enter into treaties perhaps at first to legally and fairly procure land from Native Americans. However, most famously with the Plains Indians (Lakota, Comanche, Kiowa...) who raided white settlements and caravans beginning in the early 1800's, the government attempted to make treaties to protect settlers and promote the opening up of the West to whites, a policy what would prove disasterous to the Plains Indians. Treaties, which were almost always broken due to mutual misunderstanding, allowed the government to make covert war on and take land from Native Americans legally.