We are about finding our ancestors and their extended families. We are always looking for cousins and how different families are related.
In order to facilitate finding the various links between families, we try to list as many related people in the families instead of only looking back at our specific ancestors. I believe that it adds interest to the story of say a birth record when you find that the Clerk of the Town where the birth is recorded is also a cousin of the Mother or Father or both and therefore related to the child.