hi, it must be usefull to have:
stereo Left/fx right with the 2 xlr output for live purpose.
Stereo left= guitare + amp + post fx left
Fx right= only post fx right
If 2 guitars in the band, first guitarist can playing at left and send a part of fx to the right and the other guitarist can playing at right and send a part of the fx to the left.
=> good in a mix.