Federalism is a system of government designed to protect individual rights by de-centralizing government. A de-centralized government is one of shared power. Such a system guarantees that no single individual or group has too much power.
In the United States, federalism is a dual system. This means that the power of government is split between the national government and state governments. This type of system creates checks and balances that disperse power through various channels. In creating the federal system of the United States, the Founding Fathers wanted to insure that the United States government could never be reduced to the tyrannical monarchies and patriarchies of Europe. So to ensure that the United States government remained one of, by and for the people, the power was disbursed on not only national and state levels, but it also was disbursed throughout the various branches of the government. Spreading the power across multiple branches and levels helps preserve the voice of United States citizens on an individual level. It also prevents any one person or branch from making radical changes to the United States Constitution, the document that defines the rights of the people and outlines the powers of government in the interest of protecting those rights.