The goal is to decrease costs. He’s already done a great job of that at SpaceX. From the FAQ:
Currently, tunnels are really expensive to dig, with some projects costing as much as $1 billion per mile. In order to make a tunnel network feasible, tunneling costs must be reduced by a factor of more than 10.
Dude
I was in the TBM industry for a long while.
First; The cost of funneling is a tiny fraction of the cost of the project. Cutting tunneling costs by 90% might reduce cost of project by 5%. Second; in the last 25 years productivity of TBMs has skyrocketed. Finally, Musk‘s TBM venture is similar to his failing car venture. Both industries are all about know-how, not ripe for disruption.