This question is an important one, but there will be many different answers.
I would say that the family as a social unit provides the first and most important socialization function. This is to say that it shows a child what it means to be a part of society. For example, the family loves and supports a child, disciplines and child, establishes boundaries for a child, and if all works well, sends a child into society to be a good citizen.
From this perspective, a family is the most basic social unit and arguably the most influential socialization force on a child. This is one of the most important reasons why we need strong families in our society. Without them, we invariable hurt the broader society.
Raise at least one child.