One of the most disappointing things as a reader is getting completely engrossed in a story and then it ends. You were completely into the story and following the characters and the plot finally resolves itself and it was amazing.
But then it’s over.
The elation of reading such a thrilling story is now countered by the disappointment that it’s over. So one of the greatest things an author can do is slowly pull you back out of the engrossing world of the story.
Sometimes it doesn’t require much. But the longer your story, the more it’s necessary. My all-time favorite example is the Scouring of the Shire at the end of Lord of the Rings. After following Frodo for 900 pages of walking, ring whining and orc gutting, the story can’t just end. And Tolkien gives his readers one last adventure. You know the story is over, the tension that carried you through those 900 pages is over, but you still want more. And Tolkien delivers with one last hurrah.
Ashfall by Mike Mullin delivers a more common example. It took 400 pages to reunite Alex with his family after a volcano erupts in Yellowstone and the story could have ended there. But instead we get another 50 pages that wind the story down, giving us closure (and nicely leaving room for the sequel).
These closing scenes let us emotionally process the story. After the climax, you need things to wind down a bit, to know where these characters are going to go. It doesn’t have to be long, but one last glimpse of where they’re going is gold.
PS, an epilogue is a cheap and rushed way to do it. Especially if your epilogue leaps forward in time to conveniently tie off questions like who marries who (J.K. Rowling, I’m looking at you).