Preamble to the Constitution
Two specific reasons why the Preamble was made because the first three words ("We the people") was made as all the people of the United States. The other reason was the distinctiveness of the Constitution and the government that it establishes.
The complete 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."
Present day wording
The Preamble of the Constitution means that to have a well-working system of the states, you will have to confirm the justice and make sure the people know their responsibilities. The common defense means a protection of the whole country against foreign attacks. The general Welfare means that you should promote well-being. The Blessings of ourselves and posterity means the blessing of being free and to the people around you now and in future generations. The last line saying do ordain and establish this constitution for the United States of America means to obey the laws in the Constitution.