In societies there has to be a guideline--something that regulates the conduct of people toward each other. Just think of it in smaller terms, in a family environment. Mother and Father establish rules by which children have to abide. For example, when parents tell their children that they cannot scream and yell at each other they are telling their children a (very important) rule. It regulates the behavior of the children--and so this is one factor why rules are important. The conduct between people in a society is better because of rules. Since the children from the above example learn not to scream and yell at each other do they in turn get along better with each other and not fight with each other. On a larger scale it creates the environment for a society in which screaming and yelling at each other is something that is not desirable, therefore people do keep their emotions in check and that in turn creates the same effect as with the children. People get along with each other.
Another factor is the fact that rules also establish in a society what is considered a crime and what not. Even though some crimes are literally based on the 10 commandments, others ore based on established rules. For example, take the case where an older man has sex with a young teen, say she is 15; he is in his