[sdiy] Why dual grounds?

[JH.]

> No other module has two separate grounds.

My PCBs often also have separate GND paths, even if they are not clearly 
marked as such.
In a sensitive analogue circuit, you *must* separate "Dirty GND" and "Clean 
GND" to some degree, in order to keep your signals - which are voltages 
*between*(!) signal trace and GND trace - clean. This doesn't necessarily 
mean star-grounding at the PSU (which I suppose is what Steiner did), but it 
really means to keep clean and dirty apart.
Having completely separate GNDs a bit like right hand for food, left hand 
for you know what. A good approach.
Not completely separate GNDs means more care must be taken on cleaning, i.e. 
decoupling to the right points, etc. :)

JH.