the 3118 is not a DAC its an amplifier so still needs an analog stage in front (DAC).
the simple reason we did this was to support 96 and 192 media. Combined digital dac/amp solutions are/were limited to 44/48 and not suitable. We spent a great deal of effort on a combined solution before killing it off as a project. It is however something we may return to in the future but 192 / 96 HD audio playback is important to us and our customers.
Having both dac and amp separate also provides headphone (with amp mute) and line out in parallel rather than just speaker out.