Others are just wires, like the stock Apple docks - these are basically just splitters where you get line out in the 3.5mm slot and data/power in the dock's 30-pin - but as opposed to a regular cable with both this one holds your iPod up. For some that's important in some set-ups for neatness and convenience. For others like the Pure i20, Onkyo ND-S1, Wadia 170i, etc, they have circuits in them that send out a digital stream from the iPod (the Onkyo and Wadia AFAIK only have wires from the analog pin on the dock going to the RCA analog outputs). The Pure i20 however has a DAC in it, much like other devices that are certified iDevice DACs like the iDo and some portable solutions (the names I'm not too familiar with), except they don't come in a "dock" form factor. You can always use a stock Apple dock (plus maybe a CCK when necessary with non-iDevice specified DACs), to get a digital stream out of it while on a dock.
BTW, I have a vague memory that there was an Apple dock that did have one, but I can't be too sure. Something like the iP5 dock with a Cirrus Logic chip in it or something.
Here's an iPad on a stock Apple dock, but I have to use a CCK to activate the digital audio stream as I like my amp's USB DAC enough to not blow money on an HRT iStreamer, the only one I've listened to personally, especially when this is the only iDevice I have. I'd try the i20 but two things deterred me from buying without using it first (I'm not in the US so return shipping would be hell) : I'm not sure how stable it is if I have a 9.7" tablet on it, and I'm not really a fan of DACs and CDPs using the CS4398.
Similar to the example above, here's an Android phone on a dock with a USB OTG adapter cable hooked up to the dock's output instead of the CCK as on the iPad above
Edited by ProtegeManiac - 1/1/14 at 6:38pm