The Oil Platform needs to be built on oil patches in order for oil to be harvested and refined. Horde tankers are used to build these Oil Platforms as well as return their cargo back to a Shipyard or Oil Refinery so that it can be processed and used as the overseer chooses.
The vast amounts of oil beneath the sea must be used for the construction of ships and various war machines. Because the oceans are rich with this substance, Tankers need to construct platforms where the oil is thick. Only then can the Tankers collect their dark cargo. As the Horde drew near Lordaeron and the seas became increasingly treacherous, the Platforms built by Human drillers before the Great War were destroyed.