Volunteering hours represent preparation for adulthood.
In the community, all children begin preparing for their adult roles long before they are adults. They do this by volunteering in various settings from the age of eight. All of these hours are carefully logged and observed by the elders. The Elders can know a lot about the children and their affinities and interests and abilities by where they choose to volunteer and how much, and how good at it they are.
The volunteer hours are part of an overall process of determining the assignment for each child to see where he or she fits into the community.
In school, at recreation time, and during volunteer hours, he had noticed the Elders watching him and the other Elevens. He had seen them taking notes. (Ch. 2)
After carefully tabulating hours and meeting with instructors, the elders choose a job and give every child an assignment during the Ceremony of Twelve. This is their job for life, and it cements their role in the community. This is why they spend so many years volunteering.
When they are young, they spread out their volunteering doing what their friends do and doing a little of this and a little of that. As they get older, they will focus more and more on one area. That area will basically become their job. The children who have volunteered mostly in one place know a lot already and are more likely to know what their assignment will be. Since Jonas has volunteered a little here and a little there, he has no idea what his assignment is.
Volunteering is the first step toward the assignment, which is the ushering into adulthood. Once the children get their assignments, they begin their training and no longer volunteer. The assignment itself symbolizes, quite dramatically, the end of childhood.