wow as in i cant believe the ZBC community has turned to such crap. Here an honest individual posts up a piece of work to share with you all and he gets flamed for it why? So what if he used a photo for hair he said he did, he didn’t claim to have modeled or rendered it. behold.
“done it using zbrush for sculpt ,render and photoshop for hair and compose .”
Aside from that lets put yourself in his shoes for a moment, lets say you just finished your own sculpt of some celebrity; your focus was their facial features and you didnt want to get bogged down in sculpting hair. Not in the mood, not really a finished polished piece to add to your portfolio, whatever. Now as you can imagine looking at a bald celeb that isnt bald sort of erks you and pulls away from likeness (lets face it, hair is the frame of the face & an important feature on its own) so whats the harm in comping their hair in? This isn’t for a client, or original IP it seems (i could be wronge correct me) to be just something done out of passion and pleasure and to stress the point he didn’t take credit for the hair.
Juan to answer your question, the purpose of portraits aswell as why its not rigged, rdy for animation or a full body and what it was made for.
There are plenty of reasons, first and foremost lets just sit and assume that he wanted to sculpt scarlett johanson. He said she was cute, reason enough for most of us to try our hand at recreating her. Another reason could simply be practice; i imainge by your questions that you yourself arnt much of an artist (i could be horribly wronge but im going off the assumption that if you were you would’nt be asking these questions) Most people who draw, paint, sculpt the human form or what have you arnt initially able to create a believable face or figure from scratch and without having studied it extensively. What i mean by this is that in order to create something that looks correct one has to observe and understand the them.
ESPECIALLY the human face any minor mistake can be spotted from a mile away from anyone, artsy or not! So its imparitive to practice. Most artist have thousands of portraits under their belt and most of which other people dont see.
Bah i digress, i’ve made a lengthy post and answered questions that weren’t directed at me, slap me on the wrists i know =’( but seriously first time visit in a long time and the first post i see feels so negative for something thats just so pleasant 
now then CONSTRUCTIVE CRITS!!! KEY WORD CONSTRUCTIVE!!!