Small sonic details are attended to to keep the musician in his "happy place" - if the thing between his ears isn't happy because of some "inaudible" issue, you will nonetheless hear it in his playing.
Some musicians can tolerate all manner of odd sonic distractions - we grow used to dealing with them ("I'm a professional, I'l deal with it"). But some of us (especially if they're used to everything being perfect almost all the time) have a lesser tolerance for such things. I play almost exclusively through headphones or in ears, but a couple of months ago, I was in a situation where I had to use an amp for a workshop. Being taken out of my usual bubble of sonic precision was jarring. I got through it, but it wasn't easy.
