I doubt many claim the conscious has no function. As mentioned it presumably has, to exist. Ok possibly it could arise and not be a hindrance to survival, but chances are it proved useful.
But the issue, I believe, is more around most of the things we assume are conscious are, in fact, performed by the subconscious. One has only to think of how difficult it is to learn a new skill, because the conscious is involved in making decisions, and how easy it is to excel when one is "in the zone", the conscious taking a spectator role and the subconscious doing its thing. For example, when you first learnt to talk or to read, did you not stumble over every word ? And now you don't even put the sentence together, you know what you want to say and it just comes out right (if you are lucky).
At best the conscious may issue the overall directive but recent experiments suggest the brain has made a decision before the conscious is aware it has done so; implying a even smaller role for the conscious. It thinks it is in charge, that it is the real "you", but is that just a comfortable illusion ?