Sometimes you want to strip everything away, reduce it to the barest minum. You do so in order to really focus on the heart of the matter, to remove all distractions and potential confusion about what it really is that you’re trying to say.
In the context of portraits that means a lack of everything except the face; you get close and personal with complete exposure. The idea is to capture the subject’s personality and character and show it to the world in a naked way. There is no need to be ashamed of imperfections – indeed, give me imperfections, give me wrinkles, give me worn skin, give me asymmetrical features and you will have a portrait worth returning to long after you’ve growned tired of the models’ perfectly sculptured cheek bones.