Music has no subject beyond the combinations of notes we hear, for music speaks not only by means of sounds, it speaks nothing but sound.
I don't think I've ever seen pie advertised. That's how you know it's good. They advertise ice cream and other desserts. They advertise the bejeezus out of yogurt, but I haven't seen one pie commercial.
Through writing, through that process, they realize that they become more intelligent, and more honest and more imaginative than they can be in any other part of their life.