RIAA encoding became an industry standard peculiar to vinyl technology. The grooves are cut with this RIAA'd signal and from there reconstituted or converted by the phono preamp. Why? My guess is to make LP's achieve high fidelity. Preamp makers certainly used to tout the accuracy of their RIAA conversion sections. So, assuming CD's are mastered from something other than LP's, RIAA conversion never enters the picture in CD production.
