If a musician was simply to plug their ears with earplugs, it would protect them from the loud noise but would alter they way they sound to themselves. Give it a try - put your fingers in your ears and talk. Your voice sounds much different. For a musician this is not an option so back in the days many would perform without ear protection. The consequence? Hearing loss later in life.

Digital technology has allowed musicians to protect themselves but still hear themselves (and their band) perform. The in-the-ear monitors are expensive but very much worth it when in the long run they protect their hearing.