Glad you asked. People try to argue this philosophical question all the time, what is the purpose of government? To govern you might say. Well, that's far too simplistic of an answer.
To protect the common good, you might say. Well, why? Do we need to provide health care for every citizen. Do we need to have the government provide doctors for every person? Do we need the government to hand out food to every person? Do we want a government that provides food, clothing, and shelter for every person? That would benefit the common good, while creating a society where there is no wealth, where there is no personal ownership over anything. I say no, the government does not exist to protect the common good or common wellbeing of its people.
To help individuals, you may say. How do you protect them? Do you give them a crutch to walk on by paying them for living? Do you fix wages for all citizens to protect income to a specific degree? Do you provide them assistance such as that mentioned previously? I say no. You can help and protect individuals all you want privately, we don't need to pay for the protection of our fellow man through our government. It's your own responsibility to look after yourself.
What then? First reason is to enforce and create a semblance of order. Government exists to preserve the peace, protect its citizenry from one another through laws pertaining to rape, assault, and murder with the power to enforce said laws to protect the people. It, yes, exists to create order through protection of individual freedoms and rights such as freedom to speak or worship. We require law to make certain that our fellow man and the government does not revoke our right to act as we choose and to protect the order of society.
Second reason is to defend the nation from foreign invasion. Seems pretty straightforward, but this also require that a certain degree of government protection from destruction caused by war efforts or the like, (i.e. help to rebuild areas attacked by foreign invaders such as the WTC or Pearl Harbor in WWII).
Third reason is...is...can you think of another reason for a government to truly exist? Neither can I, because there isn't one.
Our government does not exist to establish a pension system for the people. Our government does not exist to provide you with a higher education. Our government does not exist to do specific research into medicine. Our government does not exist to regulate what you can or can not put inside of your own body of your own volition. Our government does not exist to provide health care for anyone, I don't care how old or deserving you are. Our government does not exist to provide insurance to privately owned businesses or homes that are destroyed due to natural disasters, but DOES exist to rebuild those items necessary for order such as roadways, police stations, and government buildings necessary to enforce the law and to keep a united nation.
Our government exists at the whim of the people to do the will of the people for what reason? To enforce order and to protect us from foreign invasion. If we keep our government limited to those two functions, then you will see capitalism thrive without any involvement from the federal government.