What's the use of the auxiliary fuel tank on the fighter?

There is an oil pump in the auxiliary tank, which will automatically pump oil into the main tank.

As the upstairs said, with more fuel, the endurance will be improved and the combat radius will be larger.

The advantage of the auxiliary fuel tank is that it can be abandoned in close combat, thus reducing the load and improving the maneuverability of the aircraft. However, the disadvantage is that it occupies a bomb rack, and the wing loading is large, which leads to the reduction of ammunition carried by the aircraft. The mounting of the auxiliary fuel tank will reduce the starting performance of the aircraft and increase the radar reflection area.

Therefore, in view of the shortcomings of the auxiliary fuel tank, Americans who like to play with missiles and are relatively not keen on close combat invented the "protective fuel tank" (see F- 15 for more use and F- 16 for occasional use). Because the conformal fuel tank is close to the fuselage, it has little influence on the aerodynamic performance of the aircraft, and the occupation of wing loading is almost zero, and it does not occupy the bomb racks (the conformal fuel tank of F- 15 is installed on both sides of the fuselage, so there are even four bomb racks on the conformal fuel tank, two on each side).

If we don't consider the demand of close combat, there is no doubt that the American fuel tank is advanced and useful.