The preamp's purpose it to get you that high gain without forcing you to high volumes. It's an amplifier within an amplifier, if you will, and the Gain is like the volume control for the first amp. You turn your gain up or down to get the overdrive happening in that first amp. That overdriven signal is sent to the output tubes.
Some amps have a gain and a master on each channel (like my Jet City amp, and probably other high gain gain amps would.) Others, like my DeVille have just one control control for the Clean channel, and a Gain + Master for the dirty channel.
On the DeVille, what I do is level the Clean to be what I want, then set the Gain on the dirty channel to be what I want, then set the Master Volume on the Dirty channel to match the output of the Clean channel.
Preamp OD is not quite the same as overdriving your output tubes in the power section. Unfortunately, putting your power section to overdrive means means high volumes. You will have to set everything else around that point where you get the break up you want.
Preamps give you a lot more versatility that way, and you can get great sound at lower volumes and play anywhere.
I find it indespensible to have the second channel. It sucks to compromise your sound.
...so ever since then, I always hang on to the buckle.