Bobby McFerrin is a musician I have always looked up to and admired. He embodies what I endeavor to be as a performer - someone who music just flows out of. In this video, which is a masterclass at the Aarhus Vocal Festival in 2011, Bobby explains how he was "changed molecularly" when he saw the Miles Davis quintet and how he began his process of learning to improvise. He describes how he progressed in the first six years of improvisational study from "just singing without stopping" to a fully-realized ability to improvise in his own style without imitating another vocalist. He also delivers a valuable lesson about how the audience is a part of the improvisation as a listener - fascinating.
The principles Bobby discusses can certainly apply to an instrumentalist as well, but as any jazz player will tell you, the instrument is an additional barrier to creativity because you have to train your brain to operate the machinery in order to get your musical creativity to come forth on the instrument. Vocal improvisation is an important part of the development of any performer of jazz improvisation on any instrument. It is a foundational experience for musicians of all kinds, no matter what their area of concentration. The ability to improvise makes a musician more than just a regurgitator of other musician's ideas - it transforms them into a "composer in the moment", which leads to becoming a composer of the written score or the recording studio.