I am pretty sure they are used on all models, but I know I use them on mine. Springs have been a problem for decades. If you think of putting a single spring on your work bench and hitting it with a hammer, it will compress, but the force does not just push back once, it pushes back several times. This is like a car with bad shocks. Springs bounce. In earlier days they used "hairpin" springs to minimize the coils working against each other. I think Ricardo found that the number of coils affected the frequency of the bounce. The next step was to use two springs with a specific number of coils, one wound left and one wound right. Essentially, they were trying to design in a shock absorber which is easy to put on suspension, but hard to put on valves.
There are primarily three ways to use a single spring. The first is to use spring wire that provides a lot of damping. The second is to tighten the coil spacing at the bottom (and sometimes the top also.) The third is to use "Behive" shaped springs. These have become quite common in racing circles.
David