Researching "completely" a family you know may do one of several things for you or your research. You may learn something about research or something about the family you did not know before.
Recently I obtained deeds showing how a house and a farm were sold after the owner's died. In both cases, I knew all the vitals on the family. In both cases I understood the records better because I "knew" the family. That helps me understand records later when I don't know the family. Sometimes it is easier to learn about records when the family isn't as foreign to you as the records.
And in one case I learned a few things about the family that were new to me. Another reason to search for everything.