An RIAA pre-amplifier is used between a turntable/gramophone and an amplifier that is either not equipped with an RIAA/Phono input. So, you can use an RIAA when neither your turntable or amplifier is equipped with a built-in RIAA amplifier - the RIAA standard is set by the Recording Industry Association of America and is a standard for recording and playing vinyl records.
The music signal directly from a pickup is incredibly weak and requires both adaptation and additional amplification before being sent into an amplifier. For this, you need an RIAA to get the right sound from your vinyl records.
If you want better sound from your turntable it is often also a good idea to sacrifice a little money on a good RIAA amplifier, here it gives value for money, as it is usually quite sparse how great the built-in or quite cheap RIAA is in quality.
AV-Connection can provide RIAA/pickup pre-amplifiers from eg. Rega, ifi Audio, Real Cable, Thorens, Pro-Ject, Thorens og ClearAudio - With us you can always get advice and guidance on which RIAA phono stage preamplifier fits your needs and system.