Analogy: Think of a purely signal-processing view of the system as a functional view (description of what the system does) and the lower level electronics design view (with GND) as the physical solution.
If you are working purely with voltage levels (differences relative to GND or other voltages) you might be tempted to think that it is already a signal-processing view, but you can still have a separate, distinct signal-processing view that shows the flows of information only without any reference to voltages or GND.