Why did the Founding Fathers identify the purposes of the new government in the Preamble?

The purpose of our Federal Government, as found in the Preamble of the Constitution, is to "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." In order to achieve this purpose the Founding Fathers established three main principles on which our Government is based:

Inherent rights: Rights that anyone living in America has;

Self Government: Government by the people; and

Separation of Powers: Branches of government with different powers.

-Jen