Unfortunately as a liberal, the answers are never as pat as they are for the conservative or the communist. It doesn't quite fit on a bumper sticker. For example, I am not interested if education is a role for the feds or the states, but rather if government should be involved in education at all.
Federalism is a discussion for another day.
On a practical level, government's role is do the things that, because of their huge scope and society-wide impact cannot be achieved by any other means. Example of this would be the defense of the nation, the protection of the environment, regulating trade, providing a legal system, and the like. It is logical that government should perform these action, otherwise they would not be done. On a second level, the government has a role in providing for the basic health and safety of people in our society. Public health regulations are one way of doing this.