There are several reasons why the writers of the Constitution wanted to have a separation of powers for our new government. One reason had to do with our experience as British colonies. The colonists believed the British government had too much power. The colonists believed the British government felt it could do whatever it wanted to do. The writers of the Constitution did not want to create a similar situation with our new plan of government.
A second reason for having a separation of powers is that it would give each part of government a distinct job to do. No part or branch could survive on its own. By having different jobs for each branch of government, it would keep the power of government balanced. No branch would have too much power.
A third reason for having a separation of powers is that it would be easier to develop a system to control the other branches. This is known as the system of checks and balances. Each branch has the ability to control what the other branches do. This limits the power that each branch has.