Noise is anathema to modern recordings. When once you could get away with a little background hiss in a demo, or a characterful clunk or amp buzz on an album track, digital tech renders rattle, clatter, belch and hum most unwelcome. At least, such is the sophistication of the audience's ears after years of digital audio.
Any audio chain is prone to the intrusion of noise, especially in live recording, but even in the controlled environment of a studio, aberrant sounds abound. A hissing amp, a click in the mains, an asthmatic wheeze near an open mic, the roar of flatus from a chorister suffering IBS... It all adds up when multitracking.
Of course, when there's noise in a take, you'd typically retake, but that's not always possible. Live recordings, expensive session musicians, highly strung divas and more are problematic. As is dropping in SFX sourced from location recordings, working with old movie or TV soundtrack snippets, or even attempting to polish up your own back-catalogue masters.
While audio-editing software is a must in the modern studio, tough cleaning jobs demand something else, such as iZotope RX, which has been relied upon by many pro recordists since its launch in 2008. Where an audio editor will enable you to chop out noisy sections of a recording and notch such unwelcome frequencies as mains hum and ringing drum, an audio cleaner offers a more surgical approach.