If we were simply the brain we would have no need for consciousness as we would simply be robots programmed to react to things.
Isn't this what you are doing, though? Things happen, and you react to them, based on your best assessment of the situation. And if you don't have any assessment of a situation, you just pick a random choice, hope it's good, and go with it. At least that's how I do it. How do you do it?
This is not the case, we are live beings, we are consciousness, we have free will, we're not the brain. If you have ever had an outer body experience, you would realize this.
Free will just means we have a choice on how we react to our surroundings.
If you have a choice, then it's possible that you not part of those surroundings, and that you might have a mind that is spiritually/metaphysically/mumbo-jumboishly separate from the physical reality that your brain is made from. That's what makes the question of free will so significant.
Let's assume for the sake of argument that there exists an objective reality -- a real outside world that keeps existing even when you've got your eyes shut. In that case:
1) If there is "real" free will:
Sometimes (maybe not very often, and maybe only for small things...) you can
change reality by committing a choice and "magically" interfering in the ongoing chain-reaction of events since the Big Bang. Let's say that for scientific purposes there was a copy of the universe, which was identical in every respect
except that you weren't able to make that particular choice in the copy, which meant that something different could have happened, causing the paths of the two universes to diverge. Therefore, whoever or whatever is making the choice, they are not part of that causal reality. They are operating outside of it. They were able to alter the universe!
2) If there is "illusory" free will:
You think you have a choice, or it sometimes seems like there is a choice, therefore take your pick:
a) Your perception of reality is correct, so you really do have a choice, therefore go to 1).
b) Your perception has been fooled into a false belief that there's a choice. Therefore your perception of reality must also be an illusion (at least the free will part). Therefore, inside your illusion of reality, free will is real so go to 1).
c) You've 'gamed' option b) because you realise that what seemed like a choice was just an illusion, chance, decaying potassium isotopes or something similar forcing your hand. Therefore you can either play the game and make an illusory choice, or you can accept that you don't really have a choice. Which will it be?
The illusion seems inescapable -- each new, bigger reality seems to give you the illusory freedom to choose (i) whether you want to play the game or (ii) accept that the smaller reality was fake and so is this one, ad infinitum. Therefore, inescapable illusion is equivalent to real, so go to 1).
3) Real lack of free will:
You can't do #1, and you can't get to it from #2. Therefore you don't even have the illusion of free will. You're never presented with any choices, real or imagined. I'm rejecting this option out of hand because it goes completely against my illusion of reality.
Therefore free will must exist, and (barring any major logical boo-boos) we must have some sort of existence outside of the reality that we think we exist in.