Noise is, for the most part, a common mode phenomenon, which is another way of saying that the noise is mostly equally shared between the two balanced voltage outputs and will dropout of the signal when processed through a balanced device, such as a transformer or preamp. Since many professional audio environments, such as recording halls, auditoriums, and nightclubs are chock full of wiring, lights, air-conditioning, and electromechanical gear, they are also filled with electromagnetic pollution. For this reason professional audio gear is usually balanced, or at the least, balanced at the beginning of the chain where noise is more problematic, i.e. the microphone and its amplifier.
Pro Audio in the Home?
Every year we read one or two articles proclaiming how balanced audio will become mainstream. Don't bet on it, as the recent resurrection of single-ended amplifiers does not bode well for an inherently push-pull technology.
Still, there are some situations where an existing piece of equipment in your system may boast a balanced output that could be used profitably. (Because balanced outputs are used in professional applications, the design requirements are more stringent, as the loads are more severe, i.e. 600 ohms or lower. Consequently, the balanced output is usual beefier than the average unbalance; even the connectors used are much more robust and tight than any RCA jack, all of which can equal better sound.) Or maybe noise is a real problem in your home and the use of critically placed balanced links in the chain could only help. Or maybe you would like to forego the phase splitter in your push-pull amplifier and feed a push-pull signal from the beginning.
From Unbalanced to Balanced