The concept itself is as simple as its predecessor. You open the book and read the words. This time, the story is familiar, which makes it predictable and by consequence less desirable to certain readers. But there are those among us (myself included) that like to reread books just as another person might enjoy watching reruns of their favorite show or viewing an old, familiar movie.
So here's my question - why do we go back to stories we like, even though we know what's going to happen?
Some light research reveals a couple points I think are almost universal.
1. The familiar is comfortable.
Anyone who's ever dealt with a toddler will know that once they've picked a favorite book (or 2-3 favorite books), whenever you ask them what they want to read they'll go pick out one of their favorites. It doesn't matter that they've read it a hundred times before, or that they have it memorized, or even that you read it to them twice after lunch - they love that story, and they want to read it again.
That desire for the familiar doesn't just go away when we get older. We just tend to invest our time differently, which is a bit of a shame, really. Reading good books, eating good snacks, and afternoon naps. That's the life.
2. You are aware that you have changed as a person.
I didn't think the same way about Artemis Fowl when I read it last month as I did when I read it for the first time in high school. As I get older, the way I relate to the characters and how I perceive the plot changes. I can learn different things from books I haven't read in several years, and that's as good a reason to reread something as any.
There are some books that I dig out every year - the most consistent one is The Night Before Christmas. Every year on Christmas Eve. It's a tradition, and nevermind that I have the poem memorized, it's the experience of reading it out of the book (aloud and to my friends) that makes the difference.
Now, if you'll excuse me, I'm going to finish rereading Within the Sanctuary of Wings for the third time, because I really want to re-experience the wonder of discovering a previously mythical race in the mountains of fantasy Asia.