Dynamic in music is critical because it supplies the song with emotion. For instance, in the first score of Tchaikovsky's Swan Lake, the music uses the loud areas and soft areas to create a sense of the actions it corresponds with in the ballet. The music comes in at Fortissimo to show the villainous nature of Odile, the black swan, but uses a mixture of mezzo piano and subtle decrescendos to depict the sweet nature of the white swan, Odette. Without these dynamics, the music wouldn't be nearly as powerful, and would fail to capture the true expressions it attempted to convey.
