Starship is immensely useful, even much closer to home. Financially, going to Mars is not the justification for Starship (no matter how often Musk says).
Starship will enable us to bring much more mass into orbit. Communication satellites mostly. The JWST would have been easier and cheaper to construct with the payload diameter of Starship. You could even launch much larger telescopes. Commercial space stations aren't really viable yet, and there's not that much to do for Humans in LEO. That's going to change. On top of all that, there are ideas to use Starship as a point-to-point transportation system for destinations on Earth.
Starship will enable us to bring much more mass into orbit. Communication satellites mostly. The JWST would have been easier and cheaper to construct with the payload diameter of Starship. You could even launch much larger telescopes. Commercial space stations aren't really viable yet, and there's not that much to do for Humans in LEO. That's going to change. On top of all that, there are ideas to use Starship as a point-to-point transportation system for destinations on Earth.