First, it converts line power to stable DC voltages while filtering out spurious signals that could affect the audio output.

Second, the power amplifier circuit is designed so that the output voltage is proportional to the audio input voltage, even in the face of power supply variations.

A well designed amplifier tolerates a fair amount of noise and amplitude variation on the input power line. The end goal is for any residual variation to have a minimal audible effect.

For this reason, additional power conditioning is rarely if ever needed. My source of this information comes from the musical instrument and live-sound side of audio, rather than the hi-fi side. But most mainstream designers do not recommend additional power conditioning.

Of course nothing prevents people from buying additional gear if it makes them happy. And in the audiophile world, there is some interest in exploring the behavior of historical or minimalist designs that might not have a very high level of power conditioning. I'm only thinking about "mainstream" gear.