Magnetic tape can only hold so much signal.
Making matters worse for today's high-tech music styles, the bass and treble extremes of the audible spectrum of magnetic tape have even less capacity than the midrange. Loud sounds already hide noise on their own, so any increase would only mean the overall recording level would have to be turned down to prevent distortion. This means that the loud signals are not processed, so there is no chance of causing tape overload when Noise Reduction is being used. Least treatment essentially means that if there is no benefit to be gained by changing the audio signal, then don't change it . For example, suppose you want to record a solo, loud rap on a bass drum.