Check out this interview that Mathias Döpfner had with Jeff Bezos in Berlin. They cover a lot of ground, and towards the end (skip ahead to 38:00), Jeff reveals why it is so important that humans (eventually) colonize other planets.
In a nutshell: as the animals that we are, our bodies burn energy at a rate of a 100W (of which 60W is for our brains). So: we need 100 W just to be alive. But a modern, civilized human – living in a climate-controlled house, and traveling, and working – easily burns 10 times that 100W, in terms of his or her energy usage. On Earth, the number of humans will keep increasing, as will their energy needs, as civilization progresses. We have become more efficient at harnessing resources from Earth and our sun for energy, but it will not be enough. We have to find extraterrestrial resources to harness, and to generate energy from, to ensure that humans can survive another 1,000 years and more.