While I personally believe in so-called super determinism (really just determinism), an obvious implication is that we don't have a choice in whether we believe in anything, or how we react to anything. Therefore, to the extent that you're able to make a choice about how to behave given that knowledge (which is to say, not at all), you'd might as well choose to behave as if you are able to make choices. Then if you're wrong about free will, you don't act erroneously. If you're right, you never had the option to choose how to act on that belief anyway.