A human body can be in a zombie state while doing something. So a human can do something while unconscious, although we may rightfully call such a body not a real human. Conscious experience begins when that human connects to his body.
absent mindedness is still conscious tho. And sleepwalking is not the level of function im talking. You can't do your job while sleepwalking. I'm not convinced humans can do intelligent work without being conscious. No such states that i've seen anyways.
It's a very good point - that consciousness may be necessary for certain tasks like recognizing faces, counting, reading, etc. (whatever it may be).