This can be done with conventional rockets too, where the payload is fuel to be used by other craft.
I assume there are some reasons this isn't done, like the rocket equation persisting outside of Earth's atmosphere and gravity well, ie. you still have to accelerate the fuel mass if you intend on taking it with you, but taking it from launch obviates the needed complexity to rendezvous with an orbital gas station.
I assume there are some reasons this isn't done, like the rocket equation persisting outside of Earth's atmosphere and gravity well, ie. you still have to accelerate the fuel mass if you intend on taking it with you, but taking it from launch obviates the needed complexity to rendezvous with an orbital gas station.