Shimano makes both the Ultegra 6870 SS rear derailleur and the 6870 GS rear derailleur. The GS is longer then the SS, allowing the GS to work with a cassette up to 11-32. The SS only works with up to a 11-28.

The question is: Considering the GS will work with the smallest range, a 11-23, what is the advantage in the shorter SS?