Introspectively speaking, it seems to me obvious that free will is illusion. Thoughts just emerge from background causes, and actions are just thoughts about actions that we have more thoughts about committing to bringing into being (more or less the argument Sam Harris puts forth). I say this not to make a case against free will, but I think it is an intuitive one.
Evidently, even if free will (the ability to do otherwise) does not exist, consciousness (our subjective, first person experience of the world) does. Now, I've often heard it argued that consciousness is an evolved attribute that has some sort of adaptive function when it comes to the survival of organisms (think Searle's biological naturalism, for example). Arguments of the nature seem to hinge on the notion of "downward causation", where the emergent mind is able to influence the body (as opposed to epiphenomenalism). For instance, people will often argue that consciousness enables decision-making and complex thinking in living organisms, which is why it has evolved as a trait of certain organisms.
But, if one grants that free will is an illusion, what kind of adaptive benefit could consciousness really provide? What could "downward causation" even look like in the absence of free will? What would an argument for downward causation and against epiphenomenalism look like in a no-free will context?