...and the musicians around them, for timing and intonation. I always found it a bit strange and prefer the recording engineers do their job and I do mine (which could actually be as player or recorder, depending) without the players needing to hear the mix.
They are also used when playing with a click track or recording with a pre-recorded event, such as might be used to play along with a band's recording (think of a progressive rock song with orchestra parts added) or movie soundtrack.
Aside: Sony some years ago in an article about their in-ear monitors (relatively new then) said using just one actually increased hearing damage. This was in rock'ish bands, not the orchestra, I should note. The problem was, with the sound so loud in the uncovered ear, the in-ear volume was raised to compensate, leading to worse damage than if two in-ears or none were used.