I am a drummer but I play guitar for fun when I'm tired of beating skins. I'm highly experienced with drum playing/equipment, but when it comes to guitars I don't know anything.

So here's the stupid question. What is the point of using a pre-amp or a power amp, versus just using a regular amp head such as a Mesa Boogie Triple Rect. I've been doing some research and I assume that you have to use a pre amp WITH a power amp for live situations, and just a preamp to record directly. But why? What's the benefit of that verse a regular amp head? An example of a pre amp I've been looking at is a Mesa Tri Axis.

So basically what I'm asking for is a summary and pros/cons of using a regular amp head vs. using a pre-amp (with?) a power amp.
From what I understand it's mostly to get certain tones. A regular amp head has a pre amp and power amp within it, but if you were to use a seperate pre/power, you can get the tones you like of a certain pre amp and combine it with certain tones of another power amp you like. In basic schematics I think? it looks like this roughly:

guitar --> (pre amp --> power amp) --> cab

The stuff in the brackets is what is included inside a head or you can buy and use seperate ones. Or even slave amps together. Gear heads correct me if I'm wrong. Cheers!
Makes sense. The gain controls and such on a reg amp head are essentially the "pre amp".

You bring up another issue I was curious about. What is the point of slaving multiple amps? When you have identical amp heads, is it to get double the distortion? When you have two different amp heads, such as a mesa master and a marshall slave, is it to get a unique tone?