"Side effect" is perhaps not the right term, since the effect loops back into the processes; evidence for that is that we are talking about consciousness.
Do we know this is true? Could the loop back also be an illusion? From a cursory glance it seems like "free will" is massively exaggerated. However, the belief of free will seems to be healthy.
There is no largely accepted obvious place where free will emerges from, while at the same time superdeterminism is not proven as you say.
So it's fair for some to say that physics as we know it doesn't allow free will, while at the same time it's fair for others to say that obviously they have free will thus it must exist.
free will is very inefficient in a power/performance sense: most decisions are made by your subconsciousness and your free will rational self will just accept the outcome if it is barely plausible. you have to focus to even be aware that it's happening.
