Most of the drives I get are older and come from computers that were replaced, or drives that were upgraded and/or have bad sectors. Some aren't even functional. Often they come from computer repair and service companies who have been collecting them intending to wipe them but never getting around to it. I provide them with certificates logging the wiping of each one so they can even pass them on to their customers as proof of their wiping.
Like Bankie said, its more about data security and preventing identity theft than anything.