Why Do We Use Preamps?

I'm a little confused about the concept of the pre-amp. Wouldn't the most faithful reproduction of the source material involve as few steps as possible?

I do understand that some prefer the richness and coloration provided by this or that component, but for a sound that is very high in fidelity to the original sound why doesn't it make more sense to just let your amplifier do the work, with a passive preamp to adjust the volume?

Thanks,
Dusty