The preamble of the US constitution states clearly the purpose of the federal government and we all need to remember that it covers more than one thing.
"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."
Form a more perfect union - At the time the articles of confederation was not working well so the object was to make a more perfect national government. Not a perfect but more perfect. I think we are continuing to work on that idea as we are not perfect yet.
Establish Justice - One of the objectives of the federal government is to provide a level playing field through equal justice for all our citizens.
Insure domestic tranquility - Keeping the peace between citizens is never easy and that is usually been left up to the states and local governments but there have been times when those failed and federal troops had to be sent in to keep the peace.
Provide for the common defense - Some people focus on this provision as the main focus of the federal government and it is important to keep us safe from outside threats.
Promote the general welfare - Some people focus on the "general welfare" as the main effort of our government and this too is important but not more important than all the others.
Secure the blessings of Liberty - Individual liberty is an important part that is often over looked. We have the liberty to make our own choices as long as we do not obstruct the free choice of another. My right ends where the other persons nose begins is the old motto for personal liberty.
All of these proposes for the federal government are equally important and often have to be balanced one with the others. While our "Union" is still not perfect it is the best in the world. Let us keep working to prefect it.