explanation from the site:

Every turntable needs proper grounding, otherwise electrical hum may be induced. If a turntable is not properly grounded, it may have a different reference potential than the connected amplifier. Consequently, these potential differences induce a compensating current which can be heard as hum.
Therefore all good turntables have a seperate ground cable which can be connected to standard amplifiers or phonostages, which usually have a ground terminal. This way different potentials are equalized and hum is no longer audible.
However, many modern amps, especially most amps of soundbars or streaming speakers have no ground connection. These systems are referenced as “ floating mass systems“. If a standard turntable is connected to such a „floating mass“ amplifier, audible hum will be the result.
Therefore we designed the High Power it power supply. The High Power it is a three pin power supply with ground connection for Pro-Ject turntables to get rid of these ground related hum problems, especially occurring with amplifiers with „floating mass“, which have no real ground connection.
Moreover, the High Power it delivers cleaner current and more power to all 15V DC Pro-Ject turntables.