Sure, the details and specific points of a story are not real, but the ideas and people that they represent are. The point of non-fiction is generally to communicate facts, while good fiction will emphasize ideas. It is important to understand both. Fiction exists to say things that you can't just say, that people won't listen to if you state outright.
For instance, Joseph Heller could have released a statement saying, "Hey, war is bad." Uh, ok. No kidding. But instead he wrote Catch-22, which showed (in its own measly fictional way) WHY war is bad, HOW it affects people, etc.
Of course, you could get a book documenting the facts of specific wars, and that's important too, but that book, by using immense exaggeration for comedic and functional purposes, communicated something that a non-fiction book never could. If you're basing your opinion of fiction as a whole on books like Harry Potter (not that I'm undermining the importance of escapist literature; I love Harry Potter), then I can understand your confusion as to fiction's value. But look more closely at the books that have REALLY changed things in the world, and you'll realize that it was Animal Farm, not A Complete Guide to Modern Communism, that convinced people that a communist government is easily corrupted, therefore making communism a second choice to other socialist options. It's Oliver Twist and Hard Times that we look at now for reference and understanding, not A Dull and Complete Documentation of the Life of the Poor in Victorian East London. The facts of non-fiction build the reality of fiction, making it just as valid a learning tool as non-fiction.
IN RESPONSE TO YOUR RESPONSE :-D : Yes, the Rape of Nanjing was definitely hard-hitting! I was blown away by it. However, it was a different sort of gut-twisting than you would get if you read a fictionalization of the same event. And I am by no means undermining the effects of the all-the-facts-ma'am, this-is-what's-going-on approach. I think that's vastly important, in fact, reporting on such things are what I'm currently planning to do with my life, and immersing myself deeply in student loan debts to do so. :-)
Fact: Genocide is a terrible thing. You can be told this either through a non-fictional medium (like The Rape of Nanjing) or a fictional one. Someone could write a science fiction story about mass genocide in a made-up society that would also move people to hate violence and brutality. Each medium can be compelling in its own way, if done well. In fact, I think when examining an issue, a healthy balance of informative fiction and non-fiction is probably best (for me, at least).
Interesting question, by the way. I usually don't go this in depth for yahoo answers.