Many space plans propose different space vehicles for different roles and activities. There is a launcher that goes from Earth to low earth orbit or the launcher goes directly to higher orbits with lower mass. The SpaceX Super Heavy Starship should be flying to orbit by 2020 with a fully reusable rocket. It will be 2-4 more years before it is fully developed for commercial flights.
If it is flying with minimal maintenance for 100 trips then it will bring the cost of each space launch down to about $5 million per launch.
It appears to make sense for 5-10 years after the development of the commercial SpaceX Super Heavy Starship that various slightly modified versions are used in the role of orbital space tugs, fuel stations and orbit to moon and orbit to Mars vehicles.
It is similar to the old ideas about using the Space Shuttle fuel tanks back in the 1980s for space stations.
The SpaceX Super Heavy Starship can land back on Earth and would be able to act as a giant lander for the moon as well.
The Space Super Heavy Starship can bring mining equipment to the moon and make lunar fuel. This will drop costs throughout Cis-lunar system.
Fuel Costs and Fuel Requirements to Move Beyond Earth Orbit
Elon Musk has talked about the SpaceX Super Heavy Starship transferring fuel between vehicles. This was to allow many loads of fuel to be brought from Earth so that a fully fueled mission could be sent to Mars.
This also means that SpaceX Super Heavy Starships could be placed into Low Earth Orbit, High Earth Orbit, at Lagrange points, Cycler Orbits, and on the moon. They could be transferring fuel between vehicles and store fuel for other missions. Having vehicles and vehicles acting as refueling stations will greatly reduce the cost of moving around from space and not just to low earth orbit.
Going from low earth orbit to the surface of the moon takes about half of the fuel going from Earth to low earth orbit.
There could be SpaceX Super Heavy Starships placed into Lunar Cycling and Mars Cycling orbits.
Simplifying to have modifications of one vehicle for all of the roles in Space would reduce the development costs. Refueling at different points would allow for more cargo to be moved and to lower fuel costs.