I'm going to make the argument that fiction is as much grounded in this reality as a biography or textbook it is just referencing a different facet of that reality. Fiction is not an honest appeal for the reader to accept an alternate reality as fact. None of the events are considered real by writer or reader and thus do not enter into future decision making. It is instead a reflection of the mind of the writer. Because of this there is real world information to be gleaned from fiction.
Primarily fiction is a teaching tool. Metaphor and analogy allow the writer to express understandings about the world in an abstract form. Rather than teaching the specifics of a real event the writer attempts to reformulate pertinent information into a new narrative . Lessons learned in this abstracted form should be more generally applicable. Because the writer must draw from reality, the fiction is an analog of his reality. Because readers respond best to fiction that meshes with real world experience, writer attempts to make his works essentially realistic. Fiction is a retelling of reality through the lens of a writer's experiencing mind.
Second, Fiction is the product of what the writer wishes to say, or believes readers wish to hear. It is a very honest form of disseminating cultural beliefs. Reading fiction can tell you as much if not more about the attitudes of a culture than examining the real actions of those within it. The morals and lessons do not have to be adopted by the reader to be understood by the reader.
Lastly, if fiction is deemed worthy in the reader's mind it can serve as an experiential expansion. You might argue that reading non-fiction can have the same effect, but I refer back to my first point. Fiction is a more concentrated and abstracted form of experiential expression. Reading well written fiction can allow the mind to simulate many emotional and cognitive scenarios before they are encountered in real life. While basing real actions on fiction may seem ungrounded, it's how the mind works anyway. Even if someone else is not writing the fiction for us, our minds are doing it all the time anyway. We predict the outcome of action by simulating - creating a fiction.