There is a huge variety of undesirable noises out there to contaminate recordings, especially live ones.
You've got hums & buzzes of a thousand different varieties from neon signs to bad grounding, ambient noise, air conditioning, traffic and the creaks, groans and clatter from the instruments themselves to contend with, all distracting sounds when part of a recording.
Up until recently the range of techniques you could employ towards these problems was relatively limited and one-dimensional, but in the digital age it's a brave new world when it comes to combating noise!
Quieter Living Through Digital
The first amazing ‘digital age tool’ we had against noise was ‘intelligent noise reduction’ which had a learning algorithm where you could play a second of just the sound you wanted removed for the plug-in in ‘learn mode’ and then it would on the fly rewrite the digital data stream flowing through it to not include the subset of digital numbers that represented the noise component of that track. Mind-blowing!
Well this was like what we like to refer to as ‘voodoo magic fairy dust’ and it was a complete game changer since now a whole range of new possibilities were open whereas before you were just plain out of luck. Now broadband noises like nasty buzzes with five layers of overtones could be tamed as well as environments with noisy air conditioning or other ‘impossible’ noise situations.
Another more recent development is spectral repair where you can look into a sound with incredible detail and edit/copy/replace only certain areas of the sonic spectrum on a ‘microscopic’ level. Using this kind of tool we were once able to repair a piano track that had creaking piano bench noise in it while chords were sustaining! We were able to isolate each individual creak of the bench and replace that portion of the spectrum with nearby ‘non-creaky’ sound for a completely natural piano decay with no flutters or disturbances and no creaky bench. This was like magic!