Simply the ability to continue to use the deck volume faders whilst routing each deck to its own external output enabling inter-application audio mixing utilizing VB Cable virtual audio routing or similar virtual audio like JackAudio. The faders go away as soon as you enable external mixing mode and you don't have the option to route each deck in Internal mixing mode.
I'd like to be able to utilize a DAW, Ableton or Bitwig in this case to receive deck outputs but not have to control their volumes from the DAW because then I lose the ability to keep the MIDI controller locked to Traktor instead of the DAW. Sometimes I prefer to use an X1 MK3 with a custom map and still utilize the screen feedback for rotary mixing with the encoders.