They were searching for gold. Also at the time, people could get money for killing Native Americans.
The goal was to tell the rest of the world (mainly the Europeans) that the United States has official borders and that they could no longer colonize in the borders.
It also told the Europeans that the U.S would not interfere with their colonies or Europe itself.
The lord propietors, even tough the kind had full sovereignty over Carolina, were given some Powers to ensure to collect taxes and duties, establish civil structures and to keep order, also they could own some mineral field, they were avalaible to have some independent at some point.
The goverment of these colonies consisted in: A governor, a council and a populary elected assembly.
The goverments under propietary rule were similarly organized, the best know difference was who appointed the governing oficial, in these case could be the Lords Proprietors or the sovereign.
