On Dexter EZ-Lube hub/spindle assemblies, the zerk is used to repack the bearings without disassembly - as per the Dexter axle instruction manual that came with the trailer. Don't take it off.
This is not a "bearing buddy" system used for boat trailers, which uses a spring to apply a slight pressure to the grease in the hub to prevent cold water from being drawn into the bearings past the seals when warm hubs are immersed launching a boat. These bearing assy's are repacked by disassembly, cleaning and repacking.