To protect from tyranny,( when too much power gets in one persons hands), the three branches of government check and balance one another to make sure one branch does not get too powerful
As this picture shows, the three branches can check and balance one another, both the house and the senate have to pass a bill for it to become a law, the president can check the congress by rejecting a bill, but if 2/3 of the house and senate agree they can override the president.
by JR