When and why are in-ear monitors (IEMs) used together with monitor speakers?

That hybrid monitoring configuration is becoming very common. The main why and when, according to Mix Magazine, is the preferences of the band members. One member can ask for in-ears, while another can ask for speakers.

In-ear monitors are not for everyone. Drummers and keyboard players love IEMs, while guitarists and (to a lesser degree) bass players tend to dislike them. Some singers need the intimacy of direct interaction with their audience, while others can't perform without the isolation provided by in-ear monitors. With the recent introduction of “ambient” in-ear systems that blend binaural mics at performers' ears with their monitor mix, we may have reached the Holy Grail that finally tips the balance in favor of IEMs. However, for the foreseeable future, many engineers will mix a hybrid monitor system.

Another reason is to have the monitor speakers as backup for the IEMs.

Are there situations where using only one and not the other would be beneficial?

It depends on your priorities, both have their caveats.

From a mixing point of view, IEMs are far superior. Feedback is taken out of the equation from the monitoring system, each performer can have a different monitoring mix, you can implement stereo IEMs and have two channels to work with, you can do stuff that you wouldn't do in monitor speakers, like metronome or communication. Here, using in-ears only would be beneficial. (at least technically, since the ultimate benefit is to deliver an outstanding performance, and a musician might perform better with speaker monitoring systems)

From a healthy point of view, in-ear monitors, if correctly implemented, can be less damaging to your ears than monitor speaker systems. Less SPL is entering your ears, the sound doesn't mess with them as much. In this context, using in-ears exclusively is beneficial.

From the artist point of view it will be about preference. Using one and not the other would be beneficial depending on what the performer likes and is used to.

When and why are in-ear monitors (IEMs) used together with monitor speakers?

That hybrid monitoring configuration is becoming very common. The main why and when, according to Mix Magazine, is the preferences of the band members. One member can ask for in-ears, while another can ask for speakers.

In-ear monitors are not for everyone. Drummers and keyboard players love IEMs, while guitarists and (to a lesser degree) bass players tend to dislike them. Some singers need the intimacy of direct interaction with their audience, while others can't perform without the isolation provided by in-ear monitors. With the recent introduction of “ambient” in-ear systems that blend binaural mics at performers' ears with their monitor mix, we may have reached the Holy Grail that finally tips the balance in favor of IEMs. However, for the foreseeable future, many engineers will mix a hybrid monitor system.

Another reason is to have the monitor speakers as backup for the IEMs.

Are there situations where using only one and not the other would be beneficial?

It depends on your priorities, both have their caveats.

From a mixing point of view, IEMs are far superior. Feedback is taken out of the equation from the monitoring system, each performer can have a different monitoring mix, you can implement stereo IEMs and have two channels to work with, you can do stuff that you wouldn't do in monitor speakers, like metronome or communication. Here, using in-ears only would be beneficial. (at least technically, since the ultimate benefit is to deliver an outstanding performance, and a musician might perform better with speaker monitoring systems simply because s/he prefers them over in-ears)

From a healthy point of view, in-ear monitors, if correctly implemented, can be less damaging to your ears than monitor speaker systems. Less SPL is entering your ears, the sound doesn't mess with them as much. In this context, using in-ears exclusively is beneficial.

From the artist point of view it will be about preference. Using one and not the other would be beneficial depending on what the performer likes and is used to.