I have no experience of volunteering for any sort of work. As an ex-factory girl at the time there were plenty of jobs around so no need to volunteer unpaid services.
As a library assistant I just applied for the job and got it. I didn't seek out to be a library assistant or any sort of library staff, so volunteering my services didn't apply.
I can see the potential of volunteering however, especially someone who has recently graduated as a librarian and want to gain experience in a library. After leaving printing college [that's another story] nobody would take on college leavers because of a lack of experience, so I can definitely back up the need for experience. At the time nobody from the printing college mentioned about volunteering so it never occurred to the students - that's if employers would take on volunteer staff. All water under the bridge now..