Why did Spain, and later the newly independent nation of Mexico, believe they needed a "buffer" province of Texas?

Spanish colonial forces had difficulty disarming the native people and this led to a less populated Texas and a 'buffer' type state. Fierce opposition from its native people scared many immigrants away, fighting was documented between Spain and the Comanche from 1519 to a peace accord finally in 1785. The term 'buffer state' was penned to explain the space and division in-between Colonial Spain (Mexico) and the French colonial Louisiana and the territory of 'New France'.