Shorter-term, it is unclear if Starship is the right rocket for any purpose due to being spec'ed for such a wide range of purposes, and incompletely spec'ed due to faith in an iterative development process that may be totally inappropriate for developing large complex systems.
It is conceivable that a Moon station could be built and manned for a relatively small multiple of the cost of the ISS. It is conceivable such a station could be evacuated in the event of a serious failure. It is conceivable that humanity will learn interesting things about the Moon and perhaps do astronomy that would be otherwise impractical.
Landing humans on Mars and not killing them would be the highest accomplishment of sending humans to Mars. Some things will have to wait for post-humans to be created, if that ever turns out to be a good idea.