Elon has explicitly stated in interviews that one of the benefits of the design is that here on earth, it means they can be carbon neutral. I'm just relaying what he's publicly stated already. Yes, Mars ISRU is a HUGE benefit of this design, but it also means they can make their own fuel here on Earth with nothing more than some big machinery and solar panels.

I believe his exact quote was:

""" Sometimes I get criticism for, ‘why are you using combustion in rockets and you have electric cars’. There isn’t some way to make an electric rocket. I wish there was. But in the long-term, you can use solar power to extract CO2 from the atmosphere, combine it with water, and produce fuel and oxygen for the rocket. """