Michael Walzer tells us why we should care.

Question: What is the common good?

Michael Walzer: I have always been suspicious of terms in that that come in the singular. Maybe there are, maybe we should think of common goods. We want citizens, we want our children to grow up in a world where there is physical security which terrorism calls into question. Physical security is one common good. Freedom, political liberty, is another common good. A much greater degree of social equality is another common good. Those are, to my mind, the crucial common goods, and, referring back to our last question, a culture in which larger percentage of the population is engaged at a higher level, that's also a common good.