A strong federal government has the power to keep the state governments in check and to monitor the common good of all Americans. The states, in turn, keep the federal government from becoming tyrannical.
The entire structure of the United States government is based on the separation of powers. Congress, the judicial branch and the executive branch operate toward common goals with an eye toward balancing one another.
While government is not the only possible organizer of large groups of people, it is an effective one. It gathers and distributes resources for the ostensible purpose of promoting the greater good.