There are different reasons, sometimes is so they can listen their own voice, other times it includes the tick of a metronome (to keep the pitch or pace of the song), sometimes even to hear the next lines, etc. But normally is to hear their on voice, coordination with the rest of the band, and the pitch.

Once were warriors (1994), a NZ movie of pure domestic violence. I fund it in an old blockbuster store and watched several times since, the first time I did hated the movie's bad guy.

Abigail - King Diamond, can't remember correctly but says something like "Abigail was born on the 7th day of July 1777" or "dead on..."