In a word: "noise." The whole point of balanced lines is noise
rejection and it's something they're very good at. Any length of wire will act as an antenna to pick up
the random electromagnetic radiation we're constantly surrounded by: radio and TV signals as well as spurious electromagnetic noise generated by power lines, motors, electric appliances, computer monitors, and a variety of other sources. The longer the wire, the more noise it is likely to pick up. That's why balanced lines are the best choice for long cable runs. If your "studio" is basically confined to your desktop and all connections are no more than a meter or two in length, then unbalanced lines are fine—unless you're surrounded by extremely high levels of electromagnetic noise. Another place balanced lines are almost always used is in microphone cables. The reason for this is that the output signal from most microphones is very small, so even a tiny amount of noise will be relatively large, and will be amplified to an alarming degree in the mixer's high-gain head amplifier
How Do Balanced Lines Reject Nois
"Skip this section if technical details make you queasy. "
Balanced lines work on the principle of "phase cancellation": if you add two identical signals out of phase (i.e. one signal is inverted so its peaks coincide with the troughs in the other signal),
the result is ... nothing. A flat line. The signals cancel each other out.
While the desired audio signals in the hot and cold conductors are out of phase, any noise induced in the line will be exactly the same in both conductors, and thus in phase. The trick is that the phase of one signal is reversed at the receiving end of the line so that the desired audio signals become in-phase, and the induced noise suddenly finds itself out of phase. The out-of-phase noise signal is effectively canceled while the audio signal is left intact. Clever, eh?
Balanced nois cancellation
