Write about a way in which someone else's volunteer work helped you.
I work for a non-profit, religious organization. Volunteers are our backbone. They are the key to our day-to-day operations. We couldn't function without them. To write about one particular instance would be impossible as there are many. I come into contact with volunteers on a daily basis. They are amazing, dedicated people, many of which know the job better than us employees do. I don't know what I'd do without them.