This a fairly broad question and the responses you get will reflect that. I would say that the primary importance of literature is to help guide individuals and help to answer to human beings Tolstoy's two fundamental questions:  "Who we are and what shall we do?"  In my mind, I would apply both of these questions to literature in ensuring its value.  Works of literature that can answer both questions end up becoming quite powerful in addressing the human predicament.  In seeing characters in literature endure what we endure, suffer what we suffer, and battling through adversity in conditions similar to ours, I think that we gain a great deal of insight and understanding.  It is this importance that makes literature so profound and powerful.   The idea of being able to connect literature to our own lives and take potential answers from it is what makes literature so important to human beings.