Playing by pinning fingers to set keys destroys natural musical fluency – you can use any fingers.
If we can only play music using the same fingers every time, it means that muscle memory has such a critical role in how we play the piano that our musicianship is stunted. On the other hand. If we can play the same passage using different fingering each time, this means that we know how patterns of the music are formed within the structure of the keyboard.
This is fluency – the ability to effortlessly understand how patterns of tonality and rhythm are formed. It is the ability to intend every sound as we make it, to know where it lives in the tonal structure of the keyboard. True improvising – or composing in real time – becomes impossible if we rely on executing muscle-memorised finger patterns. We need to see that we can learn to improvise music as naturally as we speak, then we will play with ease and flow. Playing by pinning fingers to set keys destroys this natural musical fluency.