Humans are social animals. We could not have survived the harsh environments of those times when we needed to hunt large animals (that is when we were caveman).
But we now don't live in those environment, but it serves another purpose. It enables us to make small groups so that it is easier for us to interact with other 'groups' of society and sustain. When the elder ones die there offsprings continue that group that was created by the previous 'leader'.