I am no suspension expert, but the rate of compression for springs on cars that only have one spring does not vary, hence linear.
When two springs are employed, that is no longer the case.
The image link below is of a model remote control drift car (I didn't know there was such a thing either!), but the principle is the same.
It shows the softer spring, marked with white, compressing before the harder yellow spring does, so the rate varies.
I imagine one advantage would be allowing more rear squat to allow more weight transfer to the rear under acceleration out of corners, and more grip, without hitting the bump stops.
I guess there are advantages at the front with turn in, or maybe getting the splitter closer to the ground for more aero downforce, or hitting curbs easier, but I am only guessing.
Not sure if the images are showing a different result depending on which spring is on the bottom:
http://d2yankidori.weebly.com/upload...496309.jpg?335