Theory and practice of relativity in composing.
Those composers and musicians who only use notes / scores do not know it.
But composers who know that there is music that can be composed by playing but not by writing know it.
There is friction, slowness, in the composing work if one uses pencil and paper.
Nobody can write individual notes on a paper with the same speed as he/she can play them with both hands and feet with an instrument.
Therefore a lot of (good) music will not be composed if one uses only pencil and paper.
This is a zone that is not known by many professionals and educators in music.