NASA paid for the promise of a super cheap, high-availability, super heavy lift vehicle that could land on the moon and on Mars.

Rapidly reusable boosters and ships for refueling in orbit is the most important part of that promise, the landing on the moon and Mars part is relatively easy in comparison and has all been demonstrated by NASA themselves in the past.

Of course that's all just why they pretend to develop such systems, the true reason being American military presence anywhere on the planet within minutes.