1) The CDJ-2000 is also an HID (Human Interface Device) and can be used as a Traktor Controller.
Each CDJ-2000 soundcard can reproduce the sound of each Traktor control deck (in case there is no other soundcard installed, or in case this card is prefered as a better or more compatible solution).
2) The CDJ-2000 soundcard can also be used for recordings (although I would not choose it for that for many reasons)
I hope these 2 examples reply to your question.