"Blau gas was most famous, however, as the buoyancy compensating fuel for the LZ 127 Graf Zeppelin. It had several advantages over liquid fuels such as gasoline

"Blau gas was most famous, however, as the buoyancy compensating fuel for the LZ 127 Graf Zeppelin. It had several advantages over liquid fuels such as gasoline. Because it weighs approximately the same as air, burning it and replacing its volume with air did not lighten the airship, eliminating the need to adjust buoyancy or ballast in-flight" - https://en.wikipedia.org/wiki/Blau_gas https://en.wikipedia.org/wiki/Blau_gas If it was a hydrogen airship you could dump hydrogen to compensate, but helium needs to be reused. I'd guess they have run the numbers on the different ways to change the buoyancy and it's easier to keep the water.