What was his purpose and why?

Locke's purpose and intent was to define and promulgate the concept of a social contract between the governed and the government. His ideas in the treatise form the core of the subsequent centuries’ political dialogue about the nature, ends, and limitations of civil government. This treatise was written due to Locke's beliefs that absolute monarchy is incompatible with civil government, and that for a government to be successful, it must have the consent of the people.