When humans are going to leave the Earth-Moon system, we will need life support systems capable of running for years, without the need of too many supplies: spare parts, as well as resources such as water and air. When going to Mars, in-situ resource utilization (ISRU) might lower, or even extinguish the need for additional resources, but spare parts most will be produced on mars most unlikely (at least in the early years).

When going to even further places in the solar system the situation gets worse: first, spare parts are hard to get out there, and second, there are not many targets, that allow ISRU - and if so, the flight time (remember: you cant do ISRU in empty space) becomes longer, when going further out. To reduce this, you need higher speeds, therefore higher energies. Life-support systems that regulate them selfes like a whole biosphere might be desirable.