What this number is for
Grant applications often ask for the value of volunteer labor as an in-kind match. A dollar figure also makes an annual report easier to read: "our volunteers gave 400 hours" is vague, but "our volunteers gave $14,000 worth of time" tells a school board something concrete.
The rate is a national estimate of what an hour of volunteer time is worth in a given state, based on average wages. It is not what any individual volunteer earns, and it is not a figure you can claim as a tax deduction. Use it for reporting, not for accounting.