Why did the children leave Narnia?

Well, that depends on whether or not you're going with the actual book or what the book represents. For the actual book, Aslan is important because he is the leader of the force resisting the White Witch, and is also awesome. With the second option, the Chronicles of Narnia is symbolism for the New Testament, and Aslan is Jesus. So you would say that Jesus was important in the New Testament, wouldn't you?