The only reason I can think of is that perhaps the D3 has slightly higher quality output (with no USB input). So the E10 would just be used as a USB converter. According to the manual, it's an output, not an input, and it outputs a digital signal for the D3 to convert. I'm not sure why you'd want to do that though since the D3 probably isn't much better as a DAC than the E10 and there are plenty of 3.5mm to RCA adapters on the market, so you don't need the RCA outputs on the D3.
Edit: I suppose it could be used to give yourself a third output from the E10, but with no way to switch between the outputs, it seems a bit pointless.
Edited by LimeANite - 1/30/12 at 5:31pm