People who don't understand tablets need to let go of the idea that it's about what the device does. It's a computer. It takes an input and processes it into an output according to its programming, just like any other computer.
What seperates mainframes from desktops from tablets is the how and the where, not the what. A laptop is a desktop that can move between different desks or desk-like surfaces. Its clamshell design is inherently unbalanced, making it hard to use without a desk-like surface and hard to share with others. Its supposed need to offer all the power, drives, and connectivity of a desktop PC adds even more bulk and limits battery life, so it often remains tethered to a power cord even when not on a desk.
A tablet is a way to truly leave the desk behind, without limiting yourself to the tiny input/output area of a smartphone or iPod. Its design is lightweight and balanced, making it easy to operate one-handed or in places with no desk-like surface, such as when standing or lying down. It also makes it easy to share with others. A long battery life, combined with even longer standby time and instantaneous wake from sleep make it easy to keep detached from the wall outlet and quick to respond to a sudden use case. It's a computer which stays usable no matter what your position or location is.
What the touchscreen lacks in precision it makes up for in flexibility. The interface is customized to the application, or even a particular task within the application. If you need a keyboard, it's there. If you don't, it's gone. Even the keyboard itself is optimized to particular functions, offering a ".com" key when entering a web address, for example. Furthermore, it removes a layer of abstraction from the interface which many people find to be a barrier to computing. Rather than manipulating a pointer which manipulates objects on a screen, you touch the object on the screen directly if you want to manipulate it.
All of this is aside from the clear advantage that tablets get by embracing digital distribution even closer than conventional PCs do.