You grasp notated music if you can play with it
Is musical improvisation a must for any professionally skilled musician?
Meaning a must for classical musicians, too? Or is it a more or less pointless effort for interpreters of classical music to laboriously play around with all these pre-built phrases they would have to learn to get into something sounding like good improvisation? Something, which is, on top of that, quite rigidly bound to genres like Baroque, Jazz or Blues? Hence, in such a way specialized skills would be normally reserved for trained professionals of these genres. Moreover, the skills of musical improvisation are supposed to be only relevant, if the underlying structure of a composition is rather simple and therefore looking for inspired widening by spontaneous ideas. Which is not true for a piece by Beethoven or Mozart. Say no more…so no interpreter of such composers needs to be able to improvise.
But sometimes, questions include their answers, especially if yes and no are both true. And one aspect is ignored too often. For since the origination process of secular instrumental music no musical oevre would exist without being preceded by an ongoing trial and error with the musical material. Meaning musical improvisation must have been the creative base of most forms of notated music. So what? Yes, the invention of any music is a process of transformation of musical ideas, which have been explored in a playful and improvising manner before being notated in a score. Even the strongly determined structures of serialism needed some kind of infiltration by musically soaked gestures, which provided a conceivability of the complex textures for this kind of hyper-rational music. The very truth is indeed, there is no music without improvisation, wether it is composed music or a traditional gift being practised by local and oral traditions.
And this means by implication: If compositions have their origin in improvisation, they can only be interpreted in an adequate manner by interpreters who are well acquainted with improvising tools. Because you get an intuitive approach to deeper structures of notated music, if you have improvised with its material first. Hence this wider approach can reveal the deeper layers of a musical oevre. Listeners will have the chance to follow the hidden time-lines of the music, as if it would be discovered right now at the moment of the concert. Other approaches tend to fail while they are just scratching at the external shells of the composition.