The happy paradox of constitutional federalism is that two sets of government can protect liberty better than one. This promotion of liberty depends on a federalism of different governmental spheres laid down in the Constitution itself. The Constitution enumerates and thereby limits the powers of the federal government– basically to provide national defense, protection of interstate commerce, and a few other public goods that state and local governments cannot provide.
The states are thus left with very substantial powers. But they are forced to compete with one another in market for governance that is intensified by a few federal constitutional guarantees–those of the free flow of goods, people, and speech across state lines. As the limitation of power protects against tyranny of the federal government so does the ability of citizens to exit protect against state tyranny.
Moreover, by decentralizing most legislative responsibilities constitutional federalism addresses a fact that we must never forget: federal legislation is an exercise in central planning by temporary majorities.