I think marriage is very important because it gives a person a life-long companionship. It increases close-knit ties between families. Marriage can give you a way to extend your family name through your new generation. However, in many civilizations, people believe that marriages should be conducted within the family. (I am taking your question in this sense because its vague). The reason is that they consider it their historical tradition. In one sense they are right because marriage is the name of affiliation between two families and not only two persons. People in a family do have similar traditions and habits, therefore, after marriage the spouses will not find unfamiliarity in understanding each other. I think this is the only reason which can make it important to get married within family.