Its purpose was to give them, the lords and masters, the freedom to do as they pleased with their property, their servants and their slaves. Echoes of Magna Carta could be heard even in post-revolutionary America and they may resolve the puzzle of how, in the US, the loudest voices for liberty came from slave-owners.
Wish I could remember ref now: apparently there's evidence that more capricious government officials meant less capricious slavemasters -- even random actions by capricious officials would sometimes protect slaves, while restrained officials never would.