The shark's first dorsal fin is unusually large, taller than the pectoral fins are long. Rolling the body onto its side puts that fin into the horizontal plane where it generates lift, effectively giving the animal a larger wing at no extra tissue cost. Accelerometer and video work has estimated the saving in drag-related energy cost at around ten percent compared with swimming upright.
The behaviour turned up because biologging tags record body orientation as well as depth. It was not a rare event: tagged animals in Belize and the Bahamas spent a large fraction of their time rolled, in some records the majority.
The finding does not extend automatically to other hammerheads, whose dorsal fins are proportionally smaller, and it says nothing about hunting. Great hammerheads are also a fisheries casualty, dying at very high rates when caught, and are now listed as Critically Endangered.