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 bring diversity and special abilities.
When you recruit staff you can only get those who are looking for a job. With volunteers you have a much wider choice. You can get shop owners, university professors, computer specialists, housewives, students or pensioners. Volunteers are cost-effective . Volunteers are not paid, but organising them costs money.