The importance of that starting point cannot be overstated, for it is the foundation of whatever legitimacy our system of government can claim. It is about legitimacy. As the final member of the Bill of Rights, and the culmination of the founding period, the Tenth Amendment recapitulates the philosophy of government first set forth in the Declaration of Independence, that governments are instituted to secure our rights, that they derive their just powers from the consent of the governed. Without that consent, as manifested through constitutional ratification, power is simply illegitimate. It is the doctrine of enumerated powers, then, that gives content to the Tenth Amendment, informs its theory of legitimacy, and limits the federal government.
Power is granted in the Constitution, and thus limited by virtue of that delegation and enumeration.
The doctrine of enumerated powers was meant to be the principal line of defense against overweening government. The Bill of Rights, added two years after the Constitution was ratified, was meant as a secondary defense. Yet today the federal government exercises powers not remotely found in the Constitution, which leads lawyers and laymen alike to say, increasingly, that those powers are illegitimate. I have discussed that question at length in The Cato Handbook for Congress and elsewhere. That is the question that cries out for explanation.