Because the idea of zombies seems to make sense, and seems to, in a certain sense, be possible, I think one can use that to argue against the thesis that everything is purely physical. Now many people, I think, agree that the idea of zombies are conceivable, including people who want to be physicalists.
Kripke says that physicalists like me can't explain the 'apparent contingency' of mind-brain identities. He maintains that, if I really believed that pains are C-fibres, then I ought no longer to have any room for the thought that 'they' might come apart. His argument is that, since pains aren't identified via some contingent description, but in terms of how they feel, I have no good way of constructing a possible world, so to speak, where C-fibres are present yet pains absent.