The recipients of the nonprofit organization's services benefit from your involvement. It may be the youngsters who participate in the Big Brothers Big Sisters mentoring program whose lives improve because you give them two hours each week. Of course, the nonprofit organization receiving your volunteer labor benefits immensely. Nonprofits, almost by definition, have more needs to meet than money to meet them. The work you do saves the organization salary costs.
Volunteers also benefit in many ways.
Participating in something non-work related offers volunteers a mental vacation. You, the young lawyer, can also reap professional benefits from volunteering in nonlegal settings. And, you never know whom you might meet in a volunteer setting who just happens to need a lawyer.