All music is performed on period instruments—or authentic reproductions—from the time the music was written. Mozart, Beethoven, CPE Bach, Haydn, and Brahms never heard their music played on a modern concert grand piano, or on string instruments with nylon or metal strings. Their pianos were woodier, and their violins and cellos had gut strings; the sound was more natural, delicate, and had a subtlety almost impossible to achieve on modern instruments.
Using period instruments allows the musicians and their audiences to hear pieces from the 18th and 19th centuries in a manner that is much closer to what the composers intended when they wrote their music. The balance between piano and strings is more natural and the colors of the individual instruments are more piquant and charming.