Low Frequency signals in music typically have large amplitude / power. If these werer recorded directly into an LP, the LP Groove would not be wide enough and the Groove would rupture into the adjacent Grove.
As a solution, the RIAA ( Recording Industry Association of America ) created a standard where all signals below 500 Hz are gradually reduced before recording... The reduction / attenuation keeps increasing untill it is 20 dB of attenuation @ 50 Hz. Below 50 Hz it remains a constant attenuation of 20 dB.
To retain signal fidelity, during playback, the Phono stage Boosts by 20 dB signals between 20 Hz to 50 Hz.
From 50 Hz to 500 Hz, the boost is gradually reduced to no boost (Unity Gain) @ 500 Hz.
At the High Frequency range... signal levels are typically low, accounting for 10% or less of the audio power. If these signals were recorded at their actual (miniscule) levels, they would be difficult to detect by the cartridge & reproduce faithfully. Also when tracking such low level HF signals, the phono cartridge rubbing against the vinly grain would create Noise / Hiss. Also the Phono stage noise would be added to this HF signal.
Hence the RIAA also specified that while recording HF Signals (above 2.122 KHz, the Signals would be Boosted... The amount of boost gradually increases till its 20 dB Boost (while recording) @ 20 KHz.
Replay requires the converse, ie a CUT of 20 dB @ 20 KHz.
This cut also cuts HF Noise / hiss when reproducing the HF signal.