why is the discussion about having a free will or not even important?
In our daily lives, whether free-will is real or an illusion doesn't matter because humans base all our societies on the presumption that it exists. From free will develops the concept of responsibility (to the self and others, and in religious terms, specifically to God. This goes directly back to the story of Adam and Eve. We've been screwed because they used their free will to disobey. Of course, people disagree; but, there's always a reason why laws are created. We assume criminals have free will and intent).
Why can we talk to ourselves? Why do we have a voice in our head?
Some people who've theorized about consciousness suggest that people weren't always able to distinguish the voices in their heads from spirits. There have been, and still are, some people who claim spirits speak to them. Whether they're delusional or bullshitting is another story. Anyway, our whole species normally has the ability to distinguish. Now, can other primates do the same? They dream too.
The thing about evolution is that it's agreed that dna mutates, and that has -up until very recently- has had nothing to do with human will. "Why" does dna mutate, sometimes with disastrous results for the offspring?