text: The reason is to keep one branch from being too powerful.  One branch making all of the branches decisions is a dictatorship.  Dictatorship is when one party controls the making, execution, and interpretation of the law.  The "Federalism" idea, keeps one branch from being too powerful, and keeping one legislation from being too powerful.  That is why the House of Representatives, Senate, and  Congress all work for different aspects of the government, and why they are seperated is simply to keep one legislation from ruling all aspects of the Government.