What does the 'preamp' do exactly (in the iTunes equalizer)?

What does the ‘preamp’ do exactly (in the iTunes equalizer)? It seems to make things louder, but that can’t be all, can it?

Thanks!

Yes, it allows you to adjust the volume on a song-by-song basis.

Back in 1960, when I built my own from kits, the pre-amp was a voltage amplifier that took input from a low voltage source such as a turntable or tuner and turned it into a high voltage, but still low power. You could hook earphones directly to the output, but not a speaker. The amplifier, more properly called a power amplifier, took a high voltage, low power source and without modifying the voltage significantly, produced a high power output. What “high power” meant in this context is that you could hook to a speaker without unduly degrading the signal waves.

This was pre-transistor days, but I think it works much the same with transistors.