It appears to me that the machine can be used to make a model which a CNC machinest can then work off of? Why can't a machinest simply work off the original part?

Great question. I think the advantage is that the original part is now scanned so that it can be one place doing what ever it does and the printer can be across the planet that gives the machinist the replicated part. You could bring the printer into a project and have the scanned objects in memory and thus just make what you need- and not have to bring them with you- just bring the "powder" and a printer.