The United States of America has been a prosperous and productive nation under federalism for the past 200 years. The federal system was designed by the Framers of the Constitution to ensure democracy and liberty throughout a sparsely populated and well-distributed nation. By giving certain enumerated (and implied) powers to the national government, they were resolute to reserve all other rights to the states.
Under the Articles of Confederation, the US was a leadership and economically deficient nation. The confederal system lent too much power to the states, leaving the national government impotent. The lack of authority was replaced by the Constitution which adequately separated and appropriated power. The purpose for dividing power between the state and national level was so that tyranny and totality could be easily checked. James Madison, in Federalist No. 46 wrote that the state and national governments were made and given different powers. Madison was clearly moving to check totality of control. Alexander Hamilton concurred in Federalist No. 28 by stating that the people would sustain a balance between national and state power.
Due to its geographic size and distribution, a representative democracy was initiated. That system worked in parallel with a federal republic. Except for the Civil War, the Constitution's federal system has kept a balance of power and authority between the state and national level. Federalism has allowed localities, which differ tremendously across the country to more adequately respond to the needs and desires of their constituents. As opposed to unitary systems that work well in even homogenous societies (France, Great Britain, Japan); heterogeneous societies require governments that are more flexible and thus, responsive in dealing with the people.
The Framers of the Constitution desired a nation where liberty, life, and opportunity were available to the populace.