What do a brick, a chocolate eclair, a tennis racket and a piece of tissue paper all have in common? The answer is that they are all things that can be picked up by one hand.
The human hand is a very highly sophisticated tool that has played a very large role in the development of the species from the time when we first came down from the trees. There are scarcely any tasks we routinely perform that do not need the use of our hands. From eating and dressing to getting out of the house, working and playing. All of them are much more difficult to do if you cannot use your hands.
Going back to the list of items above, what then are the differences between them? Well, if you think about it, you pick each of them up and hold them all in different ways. For the eclair and the tissue paper your grip has to be light, otherwise you risk tearing the paper or getting cream down your shirt. The brick, however, will fall out of your hand if your grip is too light. Try using the tennis racket with it gripped between thumb and forefinger, the way you might hold the paper. You won't get many aces if you do.
When you come across a familiar object you instinctively know how best to pick it up without damaging it and holding it so it can be used. When you come across something new, you approach the job carefully, using your fingers to investigate the object until you have worked out the best way to handle it.
Picking things up is just one of the jobs these tools perform. It is also one of the simpler jobs they do, where the thumbs and fingers all work together. The more complicated tasks, like playing the piano or typing involve each digit working independently of each other. That these jobs are more difficult is self-evident, because they have to be taught instead of coming naturally. Once learnt, however, then they do become instinctual.
We also use them to communicate with each other, on both a conscious and a subliminal level. Where would we be without a hand raised in greeting or a fist punching the air in triumph? How much more difficult would it be to exactly express those feelings verbally?
Is our dexterity better than any that of any other animal? Well, that our thumb is opposable (i.e. bends in the opposite direction to our fingers) gives us a big advantage over most animals, but what about those that also have opposable digits? Simply put, our hands are the tools that we use to interact with the objects that surround us. Given that these objects are more numerous, varied and (at times) complicated than those used by, say, chimps, it follows that our control over our hands must be that much more intricate.
Without that control, life would be very much harder.