Comment by hooper

I think it's more to maintain pressure in the tank, in cases where the remaining fuel doesn't vaporize sufficiently on its own (autogenous pressurization). Otherwise, I guess atmospheric pressure or compressive force from the engines/payload could crush the tank. In the case of Starship, the fuel is moved by turbo pumps, which are integrated with the engine, and are powered by burning some of the fuel (there's a diagram of this at https://en.wikipedia.org/wiki/SpaceX_Raptor).