[Josefine]

The way I understand it is that it provides checks and balances in the execution of power between the individual states and the federal government.

Were martial law to be declared by the president of the USA without the consent of individual states, there is a provision in the US Constitution that allows an individual state to declare its independence from the federal government and to withdraw its own military back within its own borders and use them to defend their own citizens against arbitrary use of force by the federal government. They may demand the election of a new president.