For live or karaoke performances, school concerts, dancing classes, singing teachers or schools etc. To record a demonstration CD for record labels, song competitions etc...

Songs can be be purchased as Midi files that you can change or mix yourself using a GM (General Midi) keyboard or a computer, OR can be purchased as an MP3 file (an audio file you cannot change)

Midi/Karaoke/MP3 Files come with the melody (or main singing part) playing (by an instrument such as a guitar) without the melody being played, or with the melody playing softly.

For example, a professional singer would not require the melody to play along,
but the melody would be played by an instrument if the song was played in an elevator.
A karaoke machine may have the melody playing at lower volume to the other instruments to help
people sing the melody