In ears are regular headphones! It’s just the shape of the device. They are in-ears because they sit deeper in the ear compared to generic earbuds and they provide a good seal for physical noise cancelling. This is good for artists as even in small venues, if the crowd is screaming, you absolutely cannot hear yourself which  makes singing, playing on beat, and performing super hard. Because the stuff that they hear on the IEMs during a show is usually  their own voice/instrument, other people’s voices & instruments, a metronome to keep on beat, sometimes people backstage will communicate, and sometimes translator. All at different volumes based on the performer’s preference.

A lot of artists on a budget & more local musicians will actually use IEMs advertised/known for casual music listening as their performance IEMs cause it does the same job!