The postmodernists say: "What are 'facts'? What is 'world'?"

So with facts gone and the real world gone, that leaves only one purpose for language, for saying anything at all: to get others to do what you want them to do. (This is a simplification of Wittgenstein's "language-games" of the form of e.g., the Builders' Game.)

I've always maintained that Derridean postmodernism is, in actuality, a form of long-form trolling whose main purpose was to provoke traditional philosophers by baffling them or getting them to fruitlessly debate the propositions. In this it's sort of a language-game: Derrida produced words, and the philosophers reacted in just the way he intended them to when he wrote the words. Whether it was for his own amusement, to show those stiff-necked rationalists that they're not immune to cognitive or deductive traps, or whatever -- who knows? But the interesting bit is that trolling survives as a valid intention for Derrida to produce the philosophy he did, whether or not he took his own propositions seriously (though if he did, he would have to consider whether the academics he was trolling existed just in his own head).