Why did the author include a epilogue? Why did he think it was important to include one?

Ultimately, The Book Thief is framed by Death's ongoing contemplation of humanity. Death finds it impossible to weigh the value of human beings, with some capable of great malice and criminality like Hitler and others capable of great strength and bravery like Liesel and Hans: "I wanted to ask her how the same thing could be so ugly and so glorious, and its words and stories so damning and brilliant." Thus is Death haunted by humans, just as humans are haunted by Death. A jaded metaphysical being so used to dying could only be fearful of -- and, at times, amazed by -- those who live. The epilogue serves to put Death's thoughts into greater context that involves post-war experiences.