Portability mostly. There is no other device out there that is so easy and fun to use when not seated at a desk. On a train, on an airplane, in the car, it is truly amazing to use. Much better than a laptop assuming you don't need some individual feature your laptop can do but the iPad can't (which really ins't much if you're open to doing things in new ways).
Other than that, the utility of the device really depends on what extra things you want to do with it. Someone mentioned music. I'd say it makes a great digital art pad. Just browse the app store and see what interests you. It's the apps that ultimately define the device and how you use it. And fortunately, Apple's appstore has by far the most apps with the widest range of utility. Compared, of course, with Android or W7. For example, Android obviously has a lot of the standard news/rss/website apps, but lacks the more esoteric things like the aforementioned music and art apps. Games as well.