These sorts of terms are used for at least two reasons.  First, they are used because some people feel like the term "Indians" is inaccurate and offensive.  They feel that this was a name given by Columbus in error and should not be used to describe the natives of the Americas.  Second, they are used because there are those who feel they are more respectful.  They feel that using these terms recognizes the Native Americans as the original people of the Americas.  They feel that these terms implicitly recognize that the Native Americans are the people who are truly entitled to live in the Americas.