Computers just love to work with numbers, and since music and mathematics are intimately related (consider tempos, rhythmic divisions such as quarter notes, vibrato rates, the frequency of middle C, etc.), it's probably not surprising that most current electronic musical instruments contain an internal computer to do the number crunching.
In 1983, the MIDI (Musical Instrument Digital Interface) specification was introduced to better exploit the computers inside these new musical instruments and primarily to allow equipment from various manufacturers to work together. MIDI expresses musical events (notes played, vibrato, dynamics, etc.) as a common language consisting of standardized digital data. This data can be understood by MIDI compatible computers and computer-based musical instruments.
Before electronics, music was expressed exclusively as written symbols. By translating musical parameters into digital data, MIDI can express not only the types of musical events written into sheet music, but other parameters as well (such as the amount of pitch bend or degree of vibrato).