While adding a second level increases the level of work performed on the rocket, it also helps to reduce the cost of a single new rocket, bringing the costs to within reach of DIY space enthusiasts.
In the long term, the second level can be adapted for use as a space station, a habitat to the Moon or Mars, storage of more food and water for longer missions, greater shielding for solar flares from radiation, and serve as dual functions during multi space missions.
The second level increases the distance from astronauts to the fuel compartment and the rocket engine, thus enabling a more quiet and smooth flight. In space conversions will allow the joining of the rocket crew compartment second level to the spent fuel compartment in space, thus doubling or tripling available room.
This space could expand the rocket and a space station with more astronauts, tourists, and add more crew compartments for longer stays in space. It can also handle more payload such as moon or mars rocks, or serve as a hydroponics bay for the growth of food.