Why countries need a constitution?

We know that every country in our world has a constitution - the basic law of the state. Why we need it?Every society has to have some rules. If it hasn\'t, people will do whatever they want and become animals which don\'t recognize the limits of what is permissible. We have to live in a civilized society.Also the first task of the state is to keep under control and improve all areas of people\'s lives. This would be impossible without a rigid set of rights, rules and restrictions - constitution.