To prevent any disaster, and for everyone to be able to get everything they want.
Current science tells us that even if our species survives and is able to make scientific progress after the climate changes, we will always have challeges in front of us, for example:
- running out of fossil fuels completely
- running out of sand, phosphorus, rare metals
- global pandemics
- asteroid collisions
- supernovae explosions
- in 500 million years, there will not be enough CO2 to sustain photosynthesis
- in 5 billion years, the Sun will enlarge, and swallow Earth completely
- if we colonize other stars by then, they will all eventually suffer a similar fate
- red dwarves, the stars prodicted to be the longest lasting ones will too, eventually, run out of fuel, and will not be able to sustain life
- after enough time, all the matter in the universe will be sucked into black holes
- even if we could deploy techniques such as black-hole farming, even the black holes themselves would not last forever
A source of infinite energy gives us hope that we could avoid any such disasters, basically forever. Not only this, but probably we could do a lot of other cool things, if we had lots of energy, among which:
- maintain the possibility of exponential growth forever (economics)
- everyone can have their own planet, solar system, galaxy, or why not, universe
- no need for conflicts ever -> everyone can have everything they want.
- not enough room on the planet? We can create other planets, like Earth
- not enough room in the solar system? We can create other solar systems
- the end of the universe is too far away? With infinite energy, you could fly there in very little time (accelerating at 1g for enough time will lead to relativistic effects, whereby one would feel a lot less time has passed for traveling huge distances)