Besides providing a stable voltage, a regenerator will also reduce or eliminate noise and distortion that is present on the incoming AC, since it is what generates the AC that is provided to the components it is powering. Essentially it consists of an oscillator generating a 60 Hz signal (or 50 Hz in some countries) driving a high powered amplifier which in turn supplies that amplified 60 or 50 Hz signal to the connected components, and a power supply which converts the AC from the wall outlet to the DC which powers its own oscillator and amplifier.
But I understood the amplifiers require internal oscillators to operate? Why do you need to add an external oscillator?
The bandwidth limitations of the power transformer will significantly reduce the frequency components of the noise that are above a certain frequency. Noise on the incoming AC will also be reduced by filter capacitors and decoupling capacitors that will be present at various circuit locations in the design. It will also be reduced by voltage regulator circuits that are generally used in audio components, other than in the high power stages of most power amplifiers and integrated amplifiers. Finally, it will be reduced by what is known as the power supply rejection ratio of the amplification and other circuit stages which process the audio signal.
If all this processing is in the amplifier, why would it matter what you do to the power before it arrives at the amplifier? Maybe I'm missing something. But it seems to me that this is the design challenge of building amps. In theory, amplifying a signal is straightforward -- you buy an opamp and insert it into the signal path. But, addressing the departure from theory is what adds the complexity and $1000s to the price an amplifier. The designer building circuits into the amp that mitigate the problems created by the power supply. How could you market an amp that only functioned with a pristine power supply?