Answer by Kris Rosvold:
Here's the 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."
The primary puorpose of it is to call forth that the Constitution is a document of Law by virtue of Our agreement to be governed, it also makes clear that this Law is based in Principals, and to identify and call forth those Principals so that they will be considered when establishing the laws we must live by.
It also establishes clearly that We the People ordained and established this Constitution to protect Us.
By implication and logic, since We ordained it, and the Law which derives from it, We are also the ultimate authority on what is legal under the Constitution.
While SCOTUS often claims that it "doesn't have the force of law" I strongly disagree with that view, and believe that this view is entirely convenient to the purposes of those in power who have actively bribed Our Congress and intentionally gutted the Constitutional protections of our inherent Rights.
And now for a bit of educational entertainment: