What is now call the "tripartite system" is usually ascribed to French Enlightenment political philosopher known as the Baron de Montesquieu, although he didn't really use that term at all. His similar ideas were actually called "distribution of powers" by him. His ideas ended up influencing the authors of the United States Constitution in a way that there were established laws and division of duties, and, most importantly, included provisions to preserve individual liberties.
The importance of ensuring no one person or branch in government held all the power influenced the framing of the United States Constitution