the long version...
Balanced signal lines have been used by the phone companies for over 100 years. Picture a source with a transformer at one end... two wires to another transformer... then the receiver. The signal travels between the two wires, with no ground needed. Both wires have infinite impedance to ground. The definition of "balanced" lines is that both wires have the same impedance to ground.
The reason balanced lines are used is that they are immune to induced and radiated noise. Any ground referenced noise that enters the cable will be identical on both wires because of the equal impedances. The circuits at the receiving end (amplifier) are designed to amplify only the difference between the 2 wires and reject any signal (noise) that's common to both of them.
RCA cables are unbalanced... one of the signal wires is ground. Radiated and induced electrical noise is also ground referenced. Any noise that gets past the ground (RCA shield) becomes part of the signal and will be amplified at the other end.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25
Sub Stage: In process (will be Boston G310)
Amp: Boston Acoustics GTA-704 4 Ch
Rear Fill: Boston SE953 6X9's