Re: What do preamps do?
A guitar amp as you know it has 2 sections: a preamp stage and a power amp stage.
The PREAMP STAGE selecs input, applies tonal shaping, gain, and adjusts voume.
The POWER AMP stage takes the signal from the preamp and amplifies it so that you can hear it through your speaker cabinet(s).
The advantages of having seperates is, generally, a higher level of resolution, as well as the ability to mix and match different manufacturers' strong points.
For example: I have an Engl preamp, but a Marshall power amp as one of my rigs.