What’s the big deal about free will?

Either we have “free will” or we don’t.
If we don’t, then free will is an illusion, because we certainly have the perception of free will. But if we have the perception of free will, why does it matter whether or not the perception is true / valid?

Example: If I have to make a choice between A or B (i.e. not A). It may be true that my choice is pre-determined by the interaction between the environment and the cells in my body. However, my perception is that there is a possibility of choosing either (A or B). I perceive freedom to choose. I perceive that the probability of choosing A or B is greater than zero for both options. If I am wrong, and the “choice” is really an illusion, so what? I still have to participate in that illusion, don’t I?