The Tenth Amendment is meant as a limit on federal power, ceding powers not enumerated in the Constitution as those belonging to the respective states or to the people. For example, there is no power in the Constitution for any branch to regulate what we know as family law today, divorce, child support, custody, and adoption. Thus the power to regulate this area of law is left to the states. Those powers that are enumerated as within the control of the respective branches generally preempt state laws in those areas, for example, laws on immigration (with a few recent notable exceptions.)
Today's discourse on the Constitution and federalism is hopelessly tainted by the refusal of some to see the Constitution as a living document, one designed to address the needs of an evolving democracy. It is as though we all insisted, because the Bible says that God requires the sacrifice of animals, on sacrificing goats and chickens at at temple every week, or that we took a criminal to the dentist to have his teeth pulled if he assaulted someone and caused tooth loss.
Some who want to discuss the theoretical basis of federalism also have a tendency to forget their history. Prior to the Constitution, we had the Articles of Confederacy, which provided too little federal power and too many states' rights. The center did not hold. A return to such an arrangement would result in a collection of 50 separate nations with very uneasy alliances. We think it's tricky having red states and blue states now, with a painful fragmentation of what it means to be an American. I cannot imagine what would happen if the delicate balance the Constitution provides were ever to be tipped more in the direction of states' rights.
Ratified at the close of the year 1791, the Tenth Amendment was the final of the amendments that comprise what is referred to as the Bill of Rights, an addendum to the Constitution that was absolutely necessary to get it ratified by the required majorities. This amendment was written expressly to keep a rein on the power of the federal government, stating:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
To understand why the Bill of Rights was such a crucial piece, when the basic Constitution was already in place, one has to look back at the American Revolution which severed all colonial ties with Britain, and then the failed first attempt at national government, the Articles of Confederation. Escaping what many felt to be a far too powerful central government across the pond was the catalyst for many to become involved in the fight for colonial independence; this fear had been all too apparent in the Articles of Confederation, which was probably better described as a loose association or alliance of states rather than as a binding system of federal government. Under the Articles of Confederation, the central government could request, but not require many things, including taxes and troops for defense. As one might imagine, human nature being what it is, states were generally reluctant to volunteer anything that wasn't required, particularly in the area of tax revenue.
And so, although there was strong agreement among state and national leaders that the Articles of Confederation was a flop, there was also a great deal of concern that to strengthen the national government would give away states' rights and the individual freedoms for which so many Americans had sacrified so much. This concern continued through the Constitutional Convention, and was only assuaged with the Bill of Rights in place.