Which is the main purpose of the preamble of the u. s. constitution?

To lay out why the founding fathers crated the constitution 

The Preamble is the opening statement to the United States Constitution. The preamble explains the reasons why the Framers of the Constitution made our government a republic. By doing this, the founding fathers replaced the Articles of Confederation. The Preamble along with the rest of the Constitution was written over a period of about 6 weeks. The Preamble helped explain why the Constitution was written. However, it is not the law.