They have a common interest while working together on a worthwhile and meaningful project. Parents and children get to know each other with new perspectives and on different terms than they ordinarily see each other. Volunteering as a family can also change your family dynamics - try putting a teen in charge of a task from start to finish with no parental controls in place. Afterwards, however, I still wonder why they can't clean their room when I ask sometimes. Families share a common experience that they will talk about for years to come.
Kids see their parents in a work setting.
Parents help foster a work ethic in their children. Kids and parents both learn new skills - like building a deck or repairing a door. While working together, there is time to share ideas on value and ethics in a setting quite different from home. Families that work together learn about themselves. It's not always the parent that comes up with the solutions.