Do you believe in free will or determinism? Either way, imagine you are creating a life form, and that you want it to have free will. You figure out that you need to design an organ that would be responsible for this. You decide to call it the "free willer".
What would this organ do? You realize that it must make decisions, as that is what "will" allows you to do. But how will it do so? You realize that learning is involved -- that this organ needs to take into consideration past events. Oops, you have just created the deterministic willer!
Ok ok, let's back up a minute. If we don't take into consideration past events... well then the decisions it would make would have to be random. Is that what we want? Not really, random decisions won't get this new life form very far!
What other decision mechanisms could we come up with for this organ? Well it could be influenced by some predetermined set of rules or genes that get better over time, but that's just another determinism!
Ideally what we want is something that is influenced by past events (learning). Something that can improve its programming over generations (genes). And finally something that can decide to do OTHER than its deterministic inputs dictate (the free part).
Turns out that the brain marries randomness with learning and genes in a perfect way to achieve just this! We already have an organ that will purposefully introduce randomness into our thinking process so that we can come up with unique solutions and choices that are not predetermined by the universe or by our genes and past experiences (at least not exclusively).
Here's a paper I wrote about this that goes into this in more detail. The brain is sooo cool.