^^ Multiple output devices, multiple input devices, or (software allowing), outputting to more then one speaker depending on application.
Example: I use my Tritton AX Pro headset (digital) with my D2 as my primary headset, so that takes my D2's optical output. My Optical Input is connected to my PS3, eating my optical input port. My analouge ports are taken by my backup headset (Razer Barracuda AC-1).
Meanwhile, my crappy 2.1 setup is still connected to my onboard, which I use when my buddies come over (and my headphones are useless).