The FST fuel pump feeds the fuel rail(s). The volume of fuel inside the surge tank acts as a buffer to always keep the FST pump supplied with fuel. This volume is maintained by the main fuel tank's (lift) pump as well as the fuel returning from the fuel pressure regulator. For race cars and or fast road cars a surge tank is a must to prevent serious damage from fuel surge to your expenive engine. (Mixture too Lean under G-Forces as fuel pump unable to pick up fuel and supply to engine)
Installing a surge tank is also an easy way to upgrade the fuel pump of a vehicle without opening or modifying the OEM fuel tank.