Sorry but I just don't buy your premise that consciousness exerts no control over brain function or our actions. At this very moment I am using words in a very deliberate and conscious sense, creating a line of thought that would not otherwise arise. When I decide to do something, I consciously consider the risks and rewards of taking such an action. Those thoughts effect my my end decision. I'm not a robot doing all this unconsciously. But while we're here, perhaps you could tell me the survival advantage of having the illusion of volition and intention when in fact we don't. To what avail is it that we mistake the cause of our behavior for our own conscious choice? If my choice is already predetermined, wouldn't it be better to just face that and get on with the business of doing what we are already predisposed towards doing? Why this waste of time and brain power vascillating in a fantasy of conscious decisions?
First, I don't hold to choices being predetermined. I think everything is caused, and is purely the result of those causes (and quantum uncertainty) but this need not equate to predeterminism.
Secondly, the survival advantage is possibly that without such illusions, our brain could not achieve the complexity that it does. If we were not bound by those illusions then perhaps we would not be conscious, we would not be able to be conscious, and that would actually hamper the working ability and potential of our brain.
It is perhaps not that consciousness per se gives us our evolutionary advantage but rather it is perhaps merely a necessary component to develop beyond a certain point.
You can claim you are not a robot, and from one side of any illusion you are, and from the other side you are not.
Unfortunately there is no way to know whether it is illusory or not.
But working from the basic principles of science the rational conclusion is that it is an illusion... not that it doesn't exist, but that it is not what it might appear to be (hence illusory... not non-existant).
Thirdly, I don't say that the brain "exerts no control" - but rather I say that consciousness IS the working brain. It is an activity of the brain, arising from its operations. To speak of a brain controlling consciousness or vice versa is to speak of a wave controlling the medium it is in, or colour controlling its wavelength.
I.e. it is more apt, I think, to consider consciousness as a property of a working brain... the property being the activity itself.