Music can transport feelings and moods that are difficult to convey with words alone.
Improvisation opens up the possibility for a musician to move freely in a beautiful world of sound and feeling. You can add something to the existing piece and enjoy all the freedom. That means I can play loud or soft, fast or slow, with many or few notes, harmonically or charged with tension.
The beauty of this is that I am not bound by pre-set notes. That means I can enjoy the sound of the existing piece and while this creates a feeling inside of me, I can use this feeling to add another emotional note at that moment.