The basic problem with grinding in an MMO, after all, is that it doesn't require your full attention, especially if your goal is straightforward and you've done it before in one way or another. It's relatively easy to just put yourself on autopilot and do the job while your mind does something else. Listening to an audiobook is the perfect companion to this, because it fills up your mind, and leaves your hands and eyeballs itching to do something of their own.
Also, I'm a person that has trouble reading with my eyes. I can do it for short periods without any trouble, but with long books, I tend to fall asleep or get distracted very easily. Through Audible, I might have read more books with my ears than I have read with my eyes by now, and although I know some people must have the paper copy of a book in their hands, there's probably a large number of WoW players out there who find themselves not reading as much as they would like, and would love to know that there's another way to get their literary fix.