Up and running!

Good question. I have no experience with in-ear monitors, or any other kind, actually. The vast majority of my performing has been "au naturale" with no mics and with piano or orchestral backing. But, I will explain what I understand, and maybe others will chime in.

The main purpose for the monitors is for Adam to hear HIMSELF sing. If you have been to a rock concert, you know that it is so loud that you cannot hear yourself talk, sing or scream. Without monitors on stage facing the singer or in their ears, they cannot tell how they are sounding and whether they are on pitch. If you are a piano player, you hit a key and a note sounds. If you hit the right key and if the piano is in tune, your playing will be in tune. You don't have to do anything to adjust for the right pitch. The same is true with a guitar. Put your fingers on the right string between the right frets and, if your guitar is in tune, the note is in tune.

For a singer there are no measuring lines or keys or levers that play the pitch. The singer has to adjust their vocal cords and their throat muscles and their breathing to produce a note that is on the exact pitch. The slightest maladjustment, the tiniest lack of breath support or excess of breath exhaled, and the pitch will be off. A singer produces the note by sense memory, and body awareness, then monitors it by ear to make sure they hit the note correctly. If they cannot hear themselves, it's like shooting at a target in the dark.

It seems, from the signals that Adam gives his sound engineer, that the audio feed of his voice is set to sound like the audience hears it. Adam gives signals not only for loudness (point to mic, point up in the air, or move palm down) but also for sound quality (point to mic, gesture open hand like a rainbow for more brightness to the sound).

Secondarily, the other instruments are also fed into Adam's monitors. He has to hear them in order to hear the target key and pitches he is to sing. You will see him hold his palm down and wiggle his fingers to indicate the piano and usually point to the guitar players for their instruments. Each musician has their preference for what they want to hear in the monitor, and they work with the sound engineer to get that. I doubt that Adam signals the engineer for the instrument balance to the audience for Queen concerts, but he may monitor it for his own shows.

As to why Adam fiddles with the dang things more than other singers, and why more on the ballads than the up-tempo songs, I think it has to do with how he shapes his mouth to hit certain notes. To belt out those amazing glory notes, he opens his mouth wide, drops his jaw, and extends his tongue forward. This changes the shape of the ear canal which can dislodge the ear pieces. He then has to refit them. When singing a fast number without crazy high notes, he doesn't change the shape of his mouth so drastically, and the ear pieces stay put better. Other untrained singers or singers who don't sing such difficult numbers don't move their mouths excessively, and the ear pieces stay in place fine.

Adam also tends to take out at least one ear piece when he wants to hear the audience better, like for call and response.

There are also on-stage monitors facing the players and Adam, but they are not as effective for self monitoring. All the sounds in the air tend to swirl around each other and get muddy, sometimes one drowning out the other. I think they are sort of a back up system in case the in-ear monitors go wonky, as they are wont to do.