Well, if you have MIDI compatable pre-amp, you literally recall over 100 saved tones (mellow clean, huge distortion, country twang, warm jazz, classic rock) as opposed to having 2, 3, or 4 channels.

The other advantage is you can mix and match tons, say use a Mesa power amp with a Marshall pre-amp, or a VHT pre-amp, with a Carvin power-amp.
Dissonance is Bliss

Signal Chain:
Carvin CT-4
Ibanez TS-9
Carvin Quad-X
TC Electronics G-Major
Mesa/Boogie 2:90
Ear Candy BuzzBomb

Member #4 of the Carvin Club
A pre-amp can define your sound alot more...have you ever tweaked the settings on your amp a bit, and it hasn't made much difference? Then you tweak it a bit further and it sounds horrible...Well, this is where a pre-amp is handy, they are much more sensitive to the subtle changes and therefore make your sound much more defined, just how YOU want it...EQ pedals are probably the best form of pre-amp if you are now tempted