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Polygon-
Usually it's because you want more chain capacity - the two, max cog and chain capacity are related but separate. But in this case thanks to SRAMs infinite wisdom they seem to muddle the two. I can see why. It just leaves me wondering why they thought a version of the derailleur that can't work with the 11-42 was a good idea.
Howard
It's the 1X road derailure - why would you need a long cage?