What is the function of literature?

Literature can be didactic, meaning that it can be intended to teach the reader, perhaps in the form of a moral lesson. This is often quite explicitly the case with children's literature. Aesop's fables are a classic example of didactic children's literature. "The Tortoise and the Hare," for example, teaches children that it can sometimes be better to go slowly and carefully than to go quickly and complacently. "The Ant and the Grasshopper," meanwhile, teaches children that working hard and planning for the future is better than being lazy and thinking only of today.
Literature can also be polemical, meaning that it can be intended to argue a certain position or disprove another. Naomi Klein, for example, writes polemical books. In This Changes Everything, Klein argues that we must do much more to tackle the issues consequent of global warming. She also argues that we need to fundamentally rethink the economic model of capitalism if we are to meaningfully tackle climate change.
Literature is also, of course, often primarily intended to entertain. This is the primary purpose of most novels, although most novels also have a secondary purpose which might be to be didactic or polemical. J.K. Rowling's Harry Potter books, for example, are intended primarily to entertain, but they are also didactic in that they contain clear moral lessons.