https://www.cnet.com/news/spacex-exec-gwynne-shotwell-says-elon-musks-boring-company-could-create-homes-on-mars/

It's good to run multiple companies that can share resources when you have audacious goals like Elon Musk's vision of colonizing Mars. 

Musk's side project, tunnel-digging startup the Boring Company, could one day be used to create much of the living space for that Martian colony, according to SpaceX President and COO Gwynne Shotwell.