It was historically a social construct to keep family together in order to legitimise the ownership of the family wealth and powers. Also it secured more or less the survival of girls and women in their new families. But there are and were other social constructs that did the same purpose. Like e.g. family tribes, polygamy. Also some people had the habit of sending their children away to more or less loosely connected relatives or friends who formally adopted these children as part of their family. It was a method to socially bond beyond biological family ties and an effort to increase survivability of family members.