The 1992 - 1998 trucks run the power for the lift pump through the Oil Pressure Sensor (OPS). The OPSs is known for failing so that the lift pump stops lifting and strains the expensive injection pump. Also on these year trucks the lift pump only starts to prime when the engine has oil pressure (or is running). This harness is setup in a way that it starts to prime before the engine starts and helps the engine start easier. The third and final feature is that a small switch is located on the harness so you can prime the fuel filter with only one person.
This harness is also recommended for WHLP to better support the load of the more powerful lift pump.