There are so many reasons that I'm not certain it can be summed up.
Let's begin with the fact that literature is the collection of human wisdom throughout history--psychology, philosophy, theology, sociology, and history itself. It helps us see the world and others from the perspectives of others who are unlike us, and it helps us see ourselves in new ways (and through others' eyes). We read to benefit from the insights of others, to open our minds to complexities of life and ambiguities of meaning. We read to explore and better understand other cultures and beliefs, because only through reading can we put on the clothes of a Puritan and go to church with him and believe as he does, or wear the sari of the Hindu and worship as she does.
When we read, we learn to see individual bias (thanks to unfairly biased questions and unreliable narrators). Reading teaches us appreciate literature itself, to see how literature has changed history (there's a reason the intelligensia is the first group to be rounded up and shot when a new regime rises).
Literature helps us see the tragedy of history. History itself shows us the bare facts and figures, but literature gives us people who fall under history's sway.
When we read, we learn to see the admirable, the nobility, in people around us, and we learn better ways to behave.
But perhaps the most important reason I've ever thought of is this: literature teaches us that we are not alone. Through reading, we learn that others have been where we are, have felt as we feel, have believed as we believe. Paradoxically, we are unique just like everyone else. But we aren't alone. Others were here and they survived...and may have even learned from it--and so may we.