The correct answer to this open question is the following.
This would be the claim that you could make in your essay about the
purpose of government.
The purpose of the United States federal government is to rule the government in the best possible way, based on the dispositions stated in the Constitution, granting the US citizens the liberties needed, to protect them from external invasions, and create the proper political and economic scenarios so people can develop, grow, and prosper.
That would be your claim.
Let's have in mind that in the case of the United States federal government, it includes a division of powers under a system of checks and balances that don't allow one branch to be more powerful than the other two.