Through history if a single person made the law, enforced the law and judged the law, there was no law, only the opinion of that person. The Framers believed that separating making law (legislative) from enforcing law (executive) and interpreting the law (judicial) would best protect the freedom of all. This is why there are three branches of government.
The American Constitution was the first effort in world history to separate government functions between legislative, executive and judicial branches. The Framers viewed this division of power as fundamental to protecting freedom and liberty while establishing a country.
The thoughts leading to the American government’s structure can be traced over 1800 years to ancient Rome, but it was the eighteenth century French philosopher Montesquieu who clearly defined three government functions and argued that separating those functions would best protect freedom and liberty. The Constitution’s Framers agreed with Montesquieu.