Actors need bricks to play with, and in fact we rejected all the improvised fragments we had made without a plan. Improvisation without a plan is like tennis without tennis balls. ↗
I keep a guitar around while writing and will improvise music. I do this for several reasons, such as that it's fun, and sometimes it helps me with the meter. ↗
Being able to improvise is the basis for creating all characters and situations, for everything to do with performing, really. And it's good therapy as well. ↗
That's the beauty of music. You can take a theme from a Bach sacred chorale and improvise. It doesn't make any difference where the theme comes from; the treatment of it can be jazz. ↗