FWIW: I am not a rocket scientist. I'd be willing to bet it is to reduce the efficiency loss due to vacuum. The fuel tank (oxygen tank) is going to empty itsel

FWIW: I am not a rocket scientist. I'd be willing to bet it is to reduce the efficiency loss due to vacuum. The fuel tank (oxygen tank) is going to empty itself. However, you can't have a vacuum in the tank. So do you acknowledge that the rate of flow of fuel exiting the tank will be slowing down over time or do you keep the tank "full" to ensure constant pressure? Helium, is inert, less dense than oxygen and cheap. It's probably the best space filling collection of atoms, and simultaneously it's density[1] forces the oxygen further "down"[0] the fuel tank. [0] down is some direction relative to thrust. [1] density according to: "weight" generated by thrust.