An organization must award a volunteering certificate. Employees deserve it for the hours they have contributed. Most corporations go for the latter. They generally have documentation to show that you certainly have done volunteered work. The granting of a certificate will also regard one's training hours. It doesn't matter if you acquire the work internally or externally. For those who have done volunteer work, certificates are of great importance to them. It's very helpful, especially when applying for a job.
They serve as an acceptance of one's volunteer skills gained through volunteerism. When an organization acknowledges one's work, this becomes the motivation to perform well for the next time. A certificate is superior because it is strong proof that an organization appreciates your work. This is more so with volunteers, especially the young ones, who are usually not paid certificates, are inexpensive, and are a source of motivation.