Why does the US have states?

Originally, the United States were also "individual power centers". Shortly after the Revolution, the 13 colonies all considered themselves independent nations. Nobody considered themselves "American". They were a Virginian, a Pennsylvanian, or a Vermonter. They only banded together for mutual defense. This was the case for close to a century. Because settling the west happened in waves, you still had "individual power centers" forming. Each successive state was formed one or two at a time. Because they were formed from a specific group of people, they would have a unique culture. So they would need their own government. The pioneers in Kansas would have different needs from their government than the urbanites in Massachusetts. People will always want their government to represent them, not some foreign culture.

In the aftermath of their victory against Britain the individual States were all fully sovereign, and voluntarily surrendered a portion of that sovereignty in exchange for increased security and guaranteed cooperation.

Were you aware that Vermont was, de facto, an independent republic from January 15, 1777 until admission to the Union as of March 4, 1791?

No understanding of the United States is possible without a study of:

• it's Constitution (Wiki);

• the Federalist Papers (Wiki) written by Publius (the pseudonym used jointly by Alexander Hamilton, James Madison and John Jay (Wiki and Outline) under which the papers were originally published) outlining the logic behind and arguments in support of the structure created; as well as

• an understanding of the failures (Federalist Papers (Overview) 15, 16 , 17, 18, 19, 20, 21 and 22) of the Articles of Confederation under which the States first attempted to unite following victory in the Revolutionary War.

The shared goal of the Constitutional Convention was to strengthen the previously created union, while simultaneously preserving key aspects of States' Sovereignty against future infringement by the Federal Government (Federalist papers 2 through 14), and of ultimate oversight by The People.

To the People of the State of New York:

AFTER an unequivocal experience of the inefficiency of the subsisting federal government, you are called upon to deliberate on a new Constitution for the United States of America. The subject speaks its own importance; comprehending in its consequences nothing less than the existence of the UNION, the safety and welfare of the parts of which it is composed, the fate of an empire in many respects the most interesting in the world. It has been frequently remarked that it seems to have been reserved to the people of this country, by their conduct and example, to decide the important question, whether societies of men are really capable or not of establishing good government from reflection and choice, or whether they are forever destined to depend for their political constitutions on accident and force. If there be any truth in the remark, the crisis at which we are arrived may with propriety be regarded as the era in which that decision is to be made; and a wrong election of the part we shall act may, in this view, deserve to be considered as the general misfortune of mankind.

Because the United States of America is a construct of the People and of the individual States that came together to create the constitution.

Preamble

We the people of the United States, in order to form a more perfect union, establish justice, insure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity, do ordain and establish this Constitution for the United States of America.

As such the Federal Government, The People, and the individual States all are assigned particular powers and authorities in a manner intended to balance against each other, similar to how within the Federal Government itself a balance of power was intended between Legislature, Executive, and Judiciary.

Specific limits on the authority of the States are given in Article I Section 10:

No state shall enter into any treaty, alliance, or confederation; grant letters of marque and reprisal; coin money; emit bills of credit; make anything but gold and silver coin a tender in payment of debts; pass any bill of attainder, ex post facto law, or law impairing the obligation of contracts, or grant any title of nobility.

No state shall, without the consent of the Congress, lay any imposts or duties on imports or exports, except what may be absolutely necessary for executing it's inspection laws: and the net produce of all duties and imposts, laid by any state on imports or exports, shall be for the use of the treasury of the United States; and all such laws shall be subject to the revision and control of the Congress.

No state shall, without the consent of Congress, lay any duty of tonnage, keep troops, or ships of war in time of peace, enter into any agreement or compact with another state, or with a foreign power, or engage in war, unless actually invaded, or in such imminent danger as will not admit of delay.

Likewise Article IV delineates certain specific authorities, privileges, immunities, guarantees, and powers reserved to the States:

• Section 1.
  Full faith and credit shall be given in each state to the public acts, records, and judicial proceedings of every other state. And the Congress may by general laws prescribe the manner in which such acts, records, and proceedings shall be proved, and the effect thereof.

• Section 2.
  The citizens of each state shall be entitled to all privileges and immunities of citizens in the several states.

  A person charged in any state with treason, felony, or other crime, who shall flee from justice, and be found in another state, shall on demand of the executive authority of the state from which he fled, be delivered up, to be removed to the state having jurisdiction of the crime.

  No person held to service or labor in one state, under the laws thereof, escaping into another, shall, in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up on claim of the party to whom such service or labor may be due.

• Section 3.

  New states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress.

  The Congress shall have power to dispose of and make all needful rules and regulations respecting the territory or other property belonging to the United States; and nothing in this Constitution shall be so construed as to prejudice any claims of the United States, or of any particular state.

• Section 4.

  The United States shall guarantee to every state in this union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.

Finally, the Tenth Amendment of the Bill of Rights specifies how all powers and authorities otherwise unassigned by the text of the Constitution are reserved to the States and The People:

Amendment X

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.