Common mode noise reduction - in 4 words or less...
A balanced line has much less potential of picking up noise, especially on long runs with low signal level. That is why it is still in use today for microphones and in studios and other parts of the pro sound world. It does have it's issues particularly with ground loops in systems with chassis, power and signal grounds running here and there. But typically it is better. Did you know that a properly balanced isolated line can even be run a long distance without shielding and still not pick up noise? We got this idea from the phone company you know!
Tubes are inherently high impedance devices. The effects of running a high impedance signal long distances include noise and issues with frequency response due to cable capacitance and the like. So the answer was to run everything through a transformer to step it down to a lower impedance (600 ohms) and "balance" it. That way they could run long lines without all the issues just mentioned.
I will say though that those things add extra parts in the circuit, so in the particular case of home stereo components located within a couple of feet of each other, especially modern solid state ones with lower impedances and hooked together with well shielded, low capacitance cable, then, no, it is not needed and might be more simple to do without.
Is that over 100 words?
