The various modules and sensors that direct the top operations communicate via the in car network. The factory head unit uses the same network to control the amp. It is known that the head unit has some functions beyond just being a radio. Warning chimes for instance. What else, isn't for sure. Other car example: Some Caddies display the tire pressure on the RDS display.
The engine and tranny communicate over the net and go into limp in mode if the net is broken.
There are aftermarket modules that connect nonfactory headunits. Those modules also replace the lost functionality. BUT the SSR is too new and few for that market to have caught up.