1. It can be a conversation starter, especially if you're familiar with the book or the author or the genre the other person is reading.
2. You might get a good book recommendation.
3. You might save yourself some time. This has happened to me a few times: I'm planning to read a particular book, and then I learn from a trusted friend that it's not worth the paper its printed on.
4. Asking the question keeps me from falling asleep, because I'm interested in books. But start talking to me about finance or the wonderful world of remodeling, and my eyes start to glaze over.
5. What people are reading gives insight into who they are. I always find it fascinating when I find older colleagues reading escapist fiction, for instance. (I do occasionally.) Or when I find younger people intentionally reading classic literature (like Tolstoy or Dickens); that's interesting, too.
6. It opens the way to other questions. Do they read mostly contemporary authors or mostly dead authors? Fiction or non-fiction? A variety or mostly from one or two genres? How do they select the titles they read? Who are their favorite authors? What do they think about what they're currently reading? If they don't read, why not?
7. How they read is also insightful. Do they read one book at a time or several at a time? If several, is it because they are an ADD reader (like me) or another reason? Do they read slowly or quickly?
8. If you've read what they're reading, hearing them talk about it can help you evaluate your own understanding of the book or the author. It also gives you a chance for debate or agreement, further solidifying your own understanding of the title in question.
9. It beats asking them about their dog and having to listen to the exploits of Fifi for the next hour and a half.