In 1885, Washington politicians and business leaders viewed the border of Indian Territory as a wall blocking expansion and personal profit. The Five Civilized Tribes (Seminole, Cherokee, Muskogee, Choctaw, and Chickasaw) had well established communities with schools, courts and complex governments, and thriving towns. In fact, literacy among the Cherokee was higher than among the whites. But these tribal governments were in the way.
The solution was a "big government" program in social engineering: forced assimilation of Indians into the American system by dividing tribal land into "allotments". Therefore, the Dawes Commission was charged with the assimilation, and began by determining who was on the tribal membership rolls and who was not. More than a hundred years later, the work of the commission is still controversial and disagreements remain about who was and is on The Rolls.
Today, The Rolls are a critical genealogy tool for people researching their Indian ancestry. Since it is a roster of ancestors, The Rolls are also used to determine one's eligibility for tribal citizenship (a digitized version can be found here). Interestingly, this process didn't assimilate tribes, but instead made them an organic part of Oklahoma state and local government.
The following is a video documentary on The Rolls and how they affected Indian society: