Fusion is likely the end-game power gen tech for humanity, assuming no new physics (and excluding Dyson structures). For the long term, it likely will be the most useful way of generating mass amounts of electricity you can get, and access to more energy enables more possibilities of all sorts of things, enabling even things that are extremely impractical today due to their energy needs
For example, carbon capture becomes a possibility, and stuff like mass desalination. And then you could, in theory, go even more extreme with stuff like terraforming mars at human timescales, with enough energy. Of course this depends how practical and efficient fusion reactors actually would be, but with enough energy you can do so so much