From what I understand, if you're using a CDJ2000-NXS2, you can connect all your CDJs using a LAN cable and then connect the DJM900-NXS2 to your laptop using a single USB connection.
The real advantage (from what I understand) is connecting this way enables you to use Rekordbox DJ in a manner that enables the user to view the waveforms on the hardware instead of the laptop screen–a kind of advanced HID functionality w/full GUI if you will.
I haven't tried it out yet, only seen some videos of users doing it thus far. Perhaps @Pulse or @Mark can expand on this a bit (and possibly correct any errors on my behalf).
Cheers.