It is the constitutional remedy for unconstitutional federal laws.
"..it is absolutely obligatory that freedom-minded Americans refocus their attention to electing State legislators, governors, judges and sheriffs who will fearlessly defend their God-given liberties..as plainly and emphatically as I know how to say it, I am telling you: ONLY THE STATES CAN DEFEND OUR LIBERTY NOW! ..this reality means we will have to completely readjust our thinking and priorities."
http://tenthamendmentcenter.com/2012/11/14/nullification-in-one-lesson/