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

The preamble is serving two purposes.  It is fulfilling the ordinary purpose of a preamble - to act as a declarative executive summary - to set the stage by defining the philosophy that was used to compose the body of the document.  But, it is also serving as a bridge to the Declaration of Independence.  It is telling the reader that this document they are about to read (The Constitution) will state how this new government will address the grievances that were defined in the Declaration of Independence.