The framers, in wanting to create a new form of government that was not authoritarian like the monarchy that they left behind, sought to draft a constitution that did not place all of the power for decision-making into one branch of government. This is why the United States has three separate branches - executive, legislative, and judicial - each with a unique role in the decision-making process and each with some degree of "checking" power on the others (this is the system that we refer to as checks and balances). This degree of group oversight did not exist in Great Britain. In the United States, Congress is the legislative or law-making branch of government. They write the laws. The executive branch is headed by the President who is commander in chief of the armed forces and has the ability to negotiate treaties, execute the laws enacted by congress, and make various appointments subject to the approval of the senate. Essentially, this branch carries out the laws. The Supreme Court acts as the primary judicial body whose main task is to make sure that the Constitution is never violated. In other words, it acts as a review board. Through this separation, the framers created a system that minimized the threat of any one branch becoming too powerful and turning into a dictatorship which is what they felt they were seeing in Britain.
The Framers thought that this was necessary because they wanted to avoid having a government or a part of government that was too powerful.
The Framers were still worried about the idea of a part of government taking too much power. They were afraid the executive might become a monarch. On the other hand, they were afraid that the majority of the people might run roughshod over the rights of a minority (the rich).
So the decided to create a government in which neither the executive nor the legislature (nor the judicial, for that matter) could have too much power.
Living through the ordeal that imitated the American Revolution, the Framers understood that the notion of a divided government that would be able to limit a potentially harmful exercise of power would be ideal for the new republic. In seeing how King George and the British form of governments operated without any sense of check or balance, the idea of Montesquieu of dividing government into branches where each served a specific function became a critical component in the framers' thinking at the Constitutional Convention. The executive, legislative, and judicial branches possessed distinct realities that would allow government to function without intruding on the other branches or creating a system of "runaway government."
Prior to the development of the Constitution the country had been dominated by unchecked power of the British government. Americans had fought a bitter war to get themselves removed from the control of tyranny. They feared that if they were not careful in establishing a government that had a balance of power the same type of control of the people would reoccur.
The forefathers were afraid of and distrustful of power in any form and this was a precedent in their minds. They wanted a form of government that could be run by the people of a nation that could distribute power.
Initially they tried to create and use the Articles of Confederation but they proved to be too weak. The framers of the Costitution met to develop a stronger component that would distribute power and set-up a system to prevent too much power to fall under any one branch.