It is the identity for the signal, but RIAA equalization cuts the high-frequency noise that's characteristic of needles in grooves. signal -> [HF boost] -> [

It is the identity for the signal, but RIAA equalization cuts the high-frequency noise that's characteristic of needles in grooves. signal -> [HF boost] -> [needle adds HF noise] -> [HF cut] -> same signal, less HF noise As for the low end, cutting the bass helps to control the size/excursions of the grooves, which are rather close together (https://groverlab.org/hnbfpr/2019-08-06-stereo-records.html https://groverlab.org/hnbfpr/2019-08-06-stereo-records.html). Dolby B for cassette tapes is the same idea (but only for the highs).