For me, free will is the end of psychology. Because if you did something of your own free will, then there's nothing there to discover, no 'reason' why you did something, no way to analyze your behavior.
But it's not just psychology, it's the whole of science. Because, if I have free will, I can use it to move objects. Then these objects basically moved not because of a law of physics (determinism), but because of free will. It's like we pluck these objects out of the deterministic net of nature. We're like little gods, constantly intervening in nature.
The person who seeks explanations, implicitly believes in determinism. The person who believes in free will, implicitly denies the ability of science to explain anything that is touched by free will.