It's a benefit for the signal-to-noise ratio, at least in theory.
Assuming that the amount of electrical noise in the head unit is
constant, the higher the preout voltage is, the smaller percentage of
the output signal will be noise.  If you double the output voltage
while keeping the noise level the same, you'll gain about 3 dB on the
signal-to-noise ratio.  Even with a S/N of only 70, which is terribly
low by modern standards, the signal is still ten million times
stronger than the noise.

With just about any modern head unit, the signal-to-noise ratio is
already so high that the benefit of high-voltage preouts is minimal at
best.  It's become more of a marketing term than anything else.
That's why home audio is still using the old standard of about 0.7
volts for preout signals.

Scott Gardner