Separation of powers is an important principle in a constitutional republic because it helps to protect against tyranny. By dividing the functions of government into three branches, each with its own distinct powers and responsibilities, it ensures that no one branch can become too powerful and dominate the others. This helps to ensure that the government remains accountable to the people and that the rights of individuals are respected. Separation of powers also allows for greater checks and balances within the government, as each branch can act as a check on the others.