There are a ton of dirt pedals on the market these days. Many of them give you transparent overdrive and/or distortion, so that the basic tone of your amp shines through. However, some pedals are the opposite; they offer complete preamp circuits inside and not only promise more gain on tap, but an entirely new amp tone. “Amp-in-a-box” pedals like those from Wampler, Bogner, BOSS, Tech 21, MXR, and more, aim to capture the sound of another amp altogether. If you have a two-channel amplifier that has one clean and one dirty channel, this is a great way to add a “third channel” to your amp and even more tonal options to your rig.
Set your amp-in-a-box pedal as you would set another amp channel. Then when you switch your amp to your clean channel, you can also hit the preamp pedal, and rather than just sounding like your regular amp channel with an effect on it, it will sound like the amp that the pedal is emulating. This effectively gives you not just another overdrive for your pedalboard, but a third amp channel. You can also use this method with single-channel amps, if you keep the amp itself set clean.