Music theory is the structure of songs you sing. A basic familiarity with the concepts of theory will allow the singer to perform a piece of music intelligently. The strongest musicians understand both the architecture and construction of the work they're building. Singers often concentrate on the voice because they are usually evaluated for the sound they produce, not their skills of musicianship. You'll need knowledge of theory, however, to develop and practice autonomously, communicate with your fellow musicians, and interpret the meaning of the music.