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Quickly though, basically a tube amp has a lot of power that needs to go somewhere - traditionally a speaker cabinet. Without a cabinet connected to the amp, the power just circulates inside the amp, which isn't good. At all. A dummy load is another way for that power to exit the amplifier. As Unavailable said, people use dummy loads so that they can record the line output or FX Send of an amplifier (basically just the preamp signal output) directly to their interface, at which point they can use impulses. This is very popular amongst people that can't crank up their amp because of neighbors, old people, parents, etc. They can enjoy the tone of a cranked tube amp without the extreme volume of the speaker cabinet.
However, with solid-state amplifiers, a dummy load is not necessary. Likewise for an amp that has a tube preamp but a solid-state power section.
~006