The ISO Line Out box is used to provide a transformer-isolated signal from any guitar amplifier speaker output jack. One common purpose is to provide a line level signal that feeds an effect processor. The wet only output of the effect can feed another amps effect loop return or power amp with speakers to have a Dry/Wet setup or even Wet/Dry/Wet.
The ISO Line Out box is NOT a load box. In other words, the amplifier must have a speaker, or suitable load, connected at all times. The ISO Line Out box is not a speaker cabinet emulator.