The inverse RIAA playback equalization also provides greater than 15 dB of high-frequency reduction at its maximum, which greatly reduces the inherent groove noise of vinyl, which would otherwise be intolerable for music playback. It's a built-in form of noise-reduction for all vinyl recordings.
As J.B pointed out, during recording the bass frequencies are greatly reduced to prevent overcutting during disc mastering; at the same time the high frequencies are heavily boosted, so that on playback through an RIAA phono preamp, the circuit reduces the high frequencies and the groove noise along with it.
Regards,
Alan
