What is the use of CC_to_GND option in starrc extract?
Lumping all coupling capacitors (Cc) into a single capacitor to ground (Cd) is an approximation. It is useful to reduce the network complexity (and memory requirements) to speed up simulation of large networks. If you are doing timing simulations it is often reasonable to expect similar results for the transition time when Cd=M*Cc, where M is usually between 0 and 2 (if nearby nets are switching with the same supply voltage). It is of course also interesting to get Cd directly when you are interested to know the lumped capacitance of a node. 

Note: These days simulators usually have their own method of reducing parasitic complexity. This may provide better approximations than the options offered by the parasitic extraction tool.