Isn't it pretty apparent that they are demonstrating a long-range ferry capability? Let's say you need to bring in a flight of MV-22's to meet an amphibious ready group. Pick up a Marine Recon element (already embarked on an LHD), and fly off the LHD to an inland target too far for the Sea Stallions. All you'd have to do is dump the ferry tanks.
Didn't the article say the Aux tanks were INBOARD?