Originally the states were considered as a critical part of the system of checks and balances in our federal form of republic. Let's take a look at the Federalist No. 28 by Alexander Hamilton.
It may safely be received as an axiom in our political system, that the State governments will, in all possible contingencies, afford complete security against invasions of the public liberty by the national authority. Projects of usurpation cannot be masked under pretenses so likely to escape the penetration of select bodies of men, as of the people at large. The legislatures will have better means of information. They can discover the danger at a distance; and possessing all the organs of civil power, and the confidence of the people, they can at once adopt a regular plan of opposition, in which they can combine all the resources of the community. They can readily communicate with each other in the different States, and unite their common forces for the protection of their common liberty.