Free will is incompatible with (causal) determinism, and I know it. I know it because I have sound arguments for it, with compelling premises. It is good for people to know the truth about things that matter, and this is one of them. So I should be glad, for your sake and not just out of vanity, if I convinced you by one of these compelling arguments. And I would be glad.
But perhaps I shouldn't be glad if I convinced everyone, and that's for two reasons. First, there actually being compatibilists helps keep incompatibilist investigators honest and leads to a deeper understanding of the ways in which determinism precludes free will. Second, while I know that freedom is incompatible with determinism, I might be wrong. That chance is sufficiently small that it's safe for me and you to risk the cost of being wrong. But the cost of everyone getting this wrong is more than the sum of the costs of the individuals getting it wrong. Once something becomes near universally accepted, it is much harder for humankind to retreat from it.
Thus, while I want to convince you of incompatibilism, I also want there to be dissent in the epistemic community. This is something like a tragedy of the commons in the epistemic sphere.
Fortunately, human nature is such that I run only an insignificant risk of getting everyone to agree with me when I offer an argument for incompatibilism. So I can offer the arguments safely.
I chose the example of incompatibilism carefully. I wouldn't say the same thing about things that I am much more confident of, say that there is a physical world or that 2+2=4. There the risk of being wrong is so small and the level of unreasonableness in denying the claim are sufficiently high that it would be good for the epistemic community to have universal agreement. On the other hand, there are philosophical doctrines which I think are likely to be true, but where I am sufficiently unsure that I would cringe if I convinced someone.