The primary purpose of the Magna Carta was to

The document was a series of written promises between the king and his subjects that he, the king, would govern England and deal with its people according to the customs of feudal law. Magna Carta was an attempt by the barons to stop a king - in this case John - abusing his power with the people of England suffering.

The primary purpose of the Magna Carta was an attempt by the barons to stop King John from abusing his power and making the people of England suffering.