I think accuracy may be the root of some of these. Check out this site

http://easyweb.easynet.co.uk/~chrish/worms.htm

Bob Engelhardt wrote:

The recent thread about choosing gears for the drive train in a gear
hobber left me wondering. You can hob gears by driving the blank with
the hob itself. I just rented a video by Jose Rodriquez that shows how
to do it. There are also web sites showing hobbing with hob-driven
blanks. This technique seems to work well enough. What makes the
independently-driven blank process better enough to justify the highly
increased complexity?

Thanks,
Bob