What rhetorical features in the preamble reveal the purpose of the constitution? what is that purpose?

Before 1789, the U. S. had been ruled by the Articles of Confederation (1781) in fear of govt. abusing power (like British rule). In 1787, it was obvious that they needed a perfect Union and so, while protecting the independence of states, the Articles of Confederation didn't describe the powers of a federal chief executive nor a judicial system. The creation of our Constitution and present form of government was informed by these and other considerations that arose during the years of the Confederation.