Imagine a user learning vi for the first time? Press escape..and the keys are instantly re-labelled.
Imagine any other tool of any type that you're starting to learn to use, or a tool you've used for a long time but with specific key combinations you haven't used for a while--you can see notes right on the keys and beside the keys instead of looking at documentation elsewhere, with keys changing as you press modifiers.
Put together your own system keyboard shortcuts? They get labelled, too.
Think of the top of the magic mouse being a display that changes around if you wanted to go into some odd mode (granted that's a less Apple-ish thing to do), or a touchpad that does the same.
And that's just the very beginning of possibilities.
This is something that will be dramatically useful. Just as with copying, Apple is very good at taking something that people have been desperately wanting for for decades and being the first to throw enough money that hardware manufacturers are finally willing to build it. It's not a question of creating a new solution by any means, it's simply convincing other people to manufacture what until then it's been hopeless to get anyone to manufacture.
(I remember being excited about some touchscreens back in the 1980's, talking with a friend about how cool multi-touch type interfaces could be, describing things that are commonly done today--my friend informed me of the sad reality that no matter how awesome (multi-touch) user interfaces would be, that it simply would not be possible to have or even to build until some preexisting large company decided to spend millions to actually build hardware that would support it--that it didn't matter how many people wanted similar things or were wanting to buy similar things or even make similar things, that they'd not be able to get anyone to manufacture it until some company could place an order for tens of millions of "new" devices. Then and only then would we be able to create things ourselves like this as the little guy, or even as a person or company engineer something similar and be able to afford to pay any manufacturer to build it--that we were basically screwed until a HUGE company made that first big purchase. It's the same thing with this sort of keyboard--once Apple makes it, others will finally be willing to make it, and we'll finally be able to have what a lot of us have wanted for decades, and it will be possible for other companies who've previously engineered the same things to be able to afford to have it manufactured. It's not that it's anything at all novel, it's just the financial reality.)