Let’s first assume determinism is true, at least with respect to all human actions.
Next, let’s agree that we inevitably talk and think about what we could do, or could have done, even if we end up doing or having done something else. That’s what deliberation is: mapping out what we could do, predicting results, and making a decision. And that is what happens when we are deciding whether to praise or blame someone for what they did: we are assigning value to a behavior that need not have come about. Even card-carrying compatibilists do this. They may say that all behavior is determined, but when they start talking about the faculties or mechanisms in a human being from which free decisions are made, they always end up talking about the general capacities of those faculties or mechanisms, which means the wide range of decisions they are able to produce, in some range of circumstances. I submit that this ends up being disguised talk about being able to do otherwise. Free actions, according to the compatibilists, are actions that stem from faculties that would have behaved differently, had reasons for different actions been present. But that’s just to say I could have done otherwise, if I had had reason to.
So all of us talk seriously about being able to do otherwise, even though the truth is that no one ever can do otherwise. So this serious talk about free will is just so much fiction. But it is useful and beneficial fiction. By pretending that we have free will, and by holding people accountable for what they do in a free-will sort of way, we create systems of incentives and disincentives which then act as causal determinants for behavior in the future. So when you steal my coconut, and we blame you for the theft, and sentence you to cleaning the hut, we make it less likely that you or others who witness your punishment will steal coconuts in the future. It may be true — though how on earth would anyone prove it? — that a society which believes in the fiction of free will ends up with better-regulated social behavior than a society of genuine, “no talk of freedom” determinists, whether hard or soft.