You would use it just to lift the rocket fuel for manned flight. It would lift rocket fuel which would be stored in depots in orbit.

The astronauts would come on conventional rockets. But then they would be able to refuel from the orbital depots for the later stages of their flight instead of having to carry all fuel with them. Thus, you make the mission cheaper.

You still need really stable rocket fuel in order to handle those accelerations without exploding.