In general, custom molded in-ear monitors reduce the ambient sound level at the users ear drum by -26dB. This assumes a proper ear impression, excellent fit and seal and use of both IEMs; one in each ear.
By reducing the ambient stage level by -26dB, a performer does not have to compete with loud levels so they can hear themselves; for instance trying to hear vocals in a wedge OVER the rest of the band on stage. With IEMs installed in the performer’s ear, the stage level is greatly reduced (generally -26dB SPL*) thus allowing a lower level of monitoring with the IEMs... the end result is a SPL* at the performers ear drum over the entire rehearsal or show time... which the benefits of cannot be overstated.
*SPL=Sound Pressure Level