. . . Moon-Watcher felt the first faint twinges of a new and potent emotion. It was a vague and diffuse sense of envy--of dissatisfaction with his life. He had no idea of its cause, still less of its cure; but discontent had come into his soul, and he had taken one small step toward humanity.
The piano is the X factor. People have a tough time following the structures when there's no piano there, spelling it out. It makes it more easily understood, particularly to people who don't know as much about music.