Question

In which section of the Declaration of Independence Is the purpose of government described

Answer 1

The purpose of government is described in the Introduction, in this section, the  United States declares all the previous British colonies as independent and sovereign states that are no longer under the control of the Royal Crown, and that can act as an independent nation and have their own Government, which can declare war, pact alliances, accept economic treaties or do anything else a free country is able to do.  

Answer 2

The introduction or "Preamble" if you will.