What were two purposes of the mission system in California?

Spain instituted the mission system in "Alta California" in the 1760's firstly to convert the natives to Christianity, and secondly, to build a series of missions a day's walk from each other along the coast, fearing that the Russians, who were laying claims to the Pacific Northwest, would push south! The two main purposes, on deeper analysis, appear to be the same--although one religious and one political, both were an expression of Spanish power in the region that would foster the growth of Spanish culture. By the 1820's, with Mexico declaring independence from Spain, and the death of 90 percent of the mission Indians due to disease or abuse, the Mission System effectively ended.