Most U.S. domestic power supplies are not the pure clean alternating sine wave they are supposed to be. Instead your expensive HiFi equipment is fed fuzzy, spiked, triangulated wave mains power. Internet-connected smart devices, LED lighting, light dimmers, and just poor electrical household wiring in general, dumps noise and unwanted interference onto the mains power circuits that your equipment is a part of. Expanding cell phone tower coverage adds further radio frequency interference (RFI). The exponential growth and use of these devices means that the benefits of power filtration (and shielding on all HiFi cabling) are more apparent than ever.
The power supply module is a key part that determines the performance of any HiFi component. Much of the cost in building higher level more expensive components goes into the power supply. Across the price-point range of components there is a wide variation in the tolerance and ability to compensate for mains power supply noise and artifacts with generally the lower price products being the least able.
Amplifiers essentially modulate power from the internal power supply fed by the wall socket; the accuracy of matching the scaled-up output signal to the input signal largely depends on the purity and delivery of power from the power supply. Unwanted signals in the power supply itself will superimpose onto the output signal not only as noise but as dynamic artifacts. Street transformers on power poles often introduce DC voltage offset that causes transformers in HiFi components to buzz and send hum to loudspeakers.
For source components operating at lower voltages, the impact of power supply irregularities has even more impact. Benefits are as dramatic with CD players, DACs and streamers as with analog components. Digital to Analog Converters (DACS) are vulnerable to power supply noise affecting the linearity of conversion and the introduction of jitter. Also many digital devices generate their own noise that can feedback into other components. The better power filters prevent this by having individually isolated sockets rather than chaining sockets together. Even turntable motors will be affected by irregular waveforms and spikes.
Electrostatic panel loudspeakers - that are required to be plugged into the wall socket - with large surface area mimick a giant antenna picking up noise and sending it back to the amplifier via the speaker cables.
Details are revealed when reducing noise not just because of pushing down the masking effect of a noise floor threshold, but because noise superimposes itself on the signal waveform making it blurred. Removing the random effects or noise and power spikes enables all digital components to operate with more certainty and less error correction - clock timing is critical. Sound stage size, depth, and space is increased. Decaying cymbals and piano harmonics are more cleanly defined. It's easier to hear fingers on strings and the sounds of an artist performing. Dynamics are more apparent. What is normally only apparent when listening late at night (when the mains supply uis calmer) can now be enjoyed earlier in the evening.
Once you have multiple components you will need more than the two sockets in standard wall outlets espcially in older homes where outlets are far apart. Buying an inexpensive power strip with surge protection from a home improvement or office supply store is not the right tool for the job; computers do not pull the same transient power requests as music does. The basic thermistors used for surge protection set a threshold to protect computers from power spikes coming from the wall socket. Unfortunately they can't tell the difference from audio components, especially power amplifiers, from asking for more power for transient musical events. To hear the full dynamics and sound stage your system is capable of avoid cheap power strips and surge protectors.
kemela has power filters to appropriately match the budget of many systems and strongly recommend their inclusion in any system. You might very well wish you had gotten one sooner.