Here is an analogy. When playing a video game, the world is fictional and enemies are algorithmic creations. Yet, while playing that game, it is useful (that is, one plays the game better) by accepting the rules of that world and pretending as if the experiences are real at some level. Eg, "This guy is trying to kill me" vs the reality of "a microprocessor is reading state in memory and as a result is rewriting that state and some of that state shows up on my monitor as colored pixels and I'm not in actual danger."
Although I intellectually believe I'm an incredibly complex mechanism that is simply letting the laws of physics play out, it is useful while playing the game of life to utilize the evolutionary developed fiction that I do have free will.
Here is another intuition pump. Do you believe the weather has a free will, or is it just a physical phenomenon? (presumably the latter) Yet when we talk about the weather, we might say there is a 30% change of rain tomorrow. The reality is it will or it won't rain, but we can't measure the state of the system's initial conditions nor simulate its evolution accurately enough to say exactly what will happen. So we use the convenient fiction that whether it rains or not is statistical. People are too complex that we can't even calculate our own behavior, so it is a convenient fiction to act as if we have free will.