Because we are social beings, the state is natural to the person.  Therefore, the state has a positive moral function.  It is an instrument to promote human dignity, protect human rights, and build the common good.  It's purpose is to assist citizens in fulfilling their responsibility to others in society.  Since, in a large and complex society these responsibilities cannot adequately be carried out on a one-to-one basis, citizens need the help of government in fulfilling these responsibilities and promoting the common good.  According to the principle of subsidiarity, the functions of government should be performed at the lowest level possible, as long as they can be performed adequately.  If they cannot, then a higher level of government should intervene to provide help.