Background layers, extra artificial drums, sound effects, choirs, buildups, and all manner of audible additions that regular instruments cannot create, can be found in the realm of synth and midi. Synth and midi can be added to tracks via many means, but the most common is via keyboard and/or PC. Synth and midi help fill out a song and can be utilized in nearly every genre of music, and in some cases, can comprise a majority of the music altogether.
Synth and midi can completely add or alter the atmosphere of a song, and fill out a track to make it more full overall. Sometimes synth or midi can be the difference between a track sounding flat or full and complete.
…and synth and midi are not modern inventions, these things were added since the 1960s to some of the most well-known musician’s music; Bowie, Queen, almost all of disco, and much much more. It has become a common addition to a studio’s musical arsenal and in many bands and projects, it is seen as just another instrument.