Answer: To provide checks and balances of power in the government, to prevent any one branch of government from taking over too much power.
Context/detail:
The "Separation of Powers" principle was an idea embedded into the plans for American government by our founding fathers, based on their reading of Enlightenment political theory. The terminology "separation of powers" was introduced by Charles-Louis de Secondat, the Baron of Montesquieu. (Usually he's referred to as just "Montesquieu.") He wrote an important work of political theory called The Spirit of the Laws, published in 1748.
Within his treatment of how governments will function best, Montesquieu argued that executive, legislative, and judicial functions of government ought to be divided between parts of the government, so that no one person or division of the government can infringe on the overall rights of others in the government or of the members of the society overall.
