Do volunteers owe a duty of care if so why?

Yes - because they assumed a duty.

Another View: Please be more specific with the question
It totally depends on what kind of volunteering you are referring to.
A volunteer cleaning up a park or roadside owes no duty of care to anyone to keep the park or roadside clean or picked up.
On the other hand a volunteer member of a Rescue Squad or a volunteer firefighter owes a duty to perform the duties of their position to the best of their training and abilities.