Fingers that fly will make people cry
STRONG HANDS MEAN YOU CAN PLAY SOFTER, NOT JUST FORTISSIMO
Dexterity of your fingers is essentially "weightlifting" for your hands. The stronger your hands become, the more variations in sounds you can make on a piano, and the more interesting you are to listen to. Dexterity is one of the most neglected areas of training, and because of this, many people who learn to play the piano really just learn how to BANG on the piano. It is quite painful to listen to, and makes otherwise beautiful music sounds like a robot is playing it.
This mono-volume-everything-is-mezzo-forte type of playing is understandable and acceptable early on as you learn to play, but it is not the mark of a more mature pianist, and certainly not from someone who actually wants people to enjoy listening to them. EVERYONE can hear the difference, even if they cannot explain the difference in words very well - they can HEAR IT! Variance in volume is only possible when sufficient hand-strength has been developed, and like all strength-training, it does not happen overnight.
EXERCISES THAT BUILD DEXTERITY IN THE HANDS ALSO TEACH PATTERN RECOGNITION
In addition to enhancing your finger and forearm strength, dexterity exercises also teach you a very, very important part of learning the piano, Pattern Recognition. This is one of the core pillars of our core curriculum, and an essential part of learning vast amounts of music quickly. For more advanced students, we will do separate exercises specifically geared toward pattern recognition, but early on - the Hanon Exercises will give you a good grounding in pattern recognition. Believe it or not, professional musicians do not memorize individual notes, they instead memorize patterns. And these patterns are made up of anywhere between 4-4,000 bars of music at a time. We have a more useful example of this phenomenon on the Pattern Recognition page. The dexterity exercises are the first few important steps toward pattern recognition.
So in conclusion, we teach Dexterity to students for two main reasons (there are more reasons than this, but these are the two most important ones): 1. Pattern Recognition2. Increased hand and lower arm strength allows you to play a larger variety of volumes3. Without good hand dexterity, you cannot create "music" you will end up just plunking the keys.