Essentially, they serve the purpose of allowing you to take a direct audio feed from a specific source. They differ, however, from ‘regular’ headphones like those you’d use to watch a movie or stream songs, in that they aren’t tuned to enhance their sound output. What you hear in a set of in-ear monitors is your source audio in its purest form. This is a small but crucial difference for a musician. As, for example, a guitarist or bass player, you need to be able to hear how your instrument sounds to an audience. Relying on on-stage monitors can be troublesome as the sound is competing with other sounds like amplifiers, drummers and vocals. IEMs solve this by both providing a direct audio feed from the mixing desk, and also blocking out ambient noise.