Assuming we don't have free will, why do we have the illusion that we do?
Three possibilities come to mind:
Is there an evolutionary purpose?
Does it arise as a consequence of our mental activities, a sort of side effect of our thinking?
Is it given a priori (something we have to think in order to think at all)?
EDIT: Thanks for all the responses! Just one thing I saw come up a few times I'd like to address: a lot of people are asking 'Why assume this?' The answer is: it's purely rhetorical! That said, I'm happy with a well thought-out 'I dispute the premiss' answer.
If you think you can accurately diagnose my insecurities from one comment, I don't know what to tell you. My response evoking an emotional response in you does not make it in itself emotional. I wrote it sitting on the toilet because I was bored at work, not because I felt that it was especially important.