It's a good question. Lots of people don't know. While it's a common question in the service industry, how often do we think about it so bluntly?
The corollary to this is, if you ask someone what he or she wants, are you prepared to give it to that person? Because giving someone what she or he wants is often much harder than just asking a preference. Yet on the other hand, it's pretty powerful too. A guaranteed way to make a lasting connection with that person.