Here's a few links that show what kind of parts we made and examples of the 3D plaster casting pieces and also the fero arm I used. You can see that trying to machine something like a centrifugal impeller from a solid billet would be nearly impossible or at least very impractical.
http://www.sturm-inc.com/PortalData/16/Resources/brochures/PatternDevelopment_.pdf
http://www.sturm-inc.com/PortalData/16/Resources/brochures/Engineering.pdf
http://www.sturm-inc.com/PortalData/16/Resources/brochures/QualityAssurance.pdf
I have a couple of those plaster 3D printed patterns in my desk, one is a bolt complete with nut and they screw together very well. The other is a ball bearing complete with inner and outer race and balls. It spins just like a real metal bearing. The tolerances they can hold during the printing process are really amazing.