TAKING SOMETHING BEAUTIFUL, AND MAKING IT YOURS
For many musicians this is perhaps the most rewarding element of being a musician - and that is to take something already beautiful, and then making it yours through improvisation.
IMPROVISATION REQUIRES MASTERY OF MULTIPLE PILLARS
You can always "improvise," but you cannot improvise well until you have a solid mastery of the other pillars. Unlike composition which can be quasi-created without having a solid understanding of the other Pillars, improvisation is much more difficult. It is almost impossible to improvise without having a solid working knowledge of Theory, Pattern Recognition, Audiation, and obviously the Dexterity to create dynamic range and subtle changes in tempo which convey feeling and musicality. This is because improvisation is a musical elaboration within an already existing framework of musical ideas and chord progressions. If you change those things (which you do when you improvize), you need to be able to tie it to the original theme or you risk creating a soufflé that collapses on itself into a hodge-podge of nonsensical notes. Worse still is that your audience will know your improvisation stinks because they typically know the tun that you are trying to improvise on! Whereas when you have an original composition, nobody knows if you messed up or not because after all - you wrote it!