It has a unique focus on the task. Volunteers are local. They come from and serve the local community. They know the needs and the resources. And when disaster strikes, they are already there.
Volunteers are cost-effective.
Volunteers are not paid, but organizing them costs money. Even so, work done by volunteers has been proven 2 to 8 times as cost-effective as work done by staff. In organized volunteering, there is need for professionals, too. In other words, to facilitate an organized outlet for people's wish to contribute to their communities or those of others require support structures and an institutional framework for action. I believe that there is fairly broad agreement on the nature of benefits that volunteering bring, and will not spend much time listing them.