I think you're confusing In Ear Monitors which are well-fitting earphones that can act as a substitute for monitor speakers with ear plugs which, when properly designed, attenuate the sound reaching your ears by a specifc amount and help protect your hearing.
Using IEMs can greatly reduce stage wash and result in a much cleaner sounding FOH mix. Taken to an extreme, you can end up with an almost silent stage and keep the levels in the IEMs safely low as well.
For hearing protection, the good ear plugs are properly designed to attenuate pretty evenly across the audible frequencies so everything sound right, just quieter. I have a set of
ETYMOTIC ER20 which have served me well for several years but I've heard good things about other brands as well.