Free will is a very useful heuristic that lets you quickly determine when to stop looking for the root cause of events that interest you. When a stone hits you

Free will is a very useful heuristic that lets you quickly determine when to stop looking for the root cause of events that interest you. When a stone hits you - you want to attack the person that threw it at you and not the stone or that person's ancestors. So you divide the world into agents with free will and objects without. And stop asking "why" when you encounter the first agent in the root-cause chain. It's a very effective heuristic, so people are strongly persuaded it's real.