Need a very patient model to sit there while you practice the basics of butterfly lighting, short lighting, and rembrandt lighting? One that never gets tired of posing for you? A mannequin head will never complain.
Portrait Lighting Patterns
Keep in mind this is a quick and dirty solution for practicing the 5 basic lighting patterns. If you want to get into metering for skin tones, learning about how different intensities of hairlight affect different hair tones (blonde vs black, for instance), and practicing the subtleties of specular highlights and catchlight positions, definitely spend a few bucks on a few mannequin heads with varying skin tones, etc. Personally, I think once you’ve learned the basic patterns, you should go straight to using real people for the things you’d use a ridiculously realistic head for.