Colonizing space is our best hope for saving the Earth and solving its problems.
Human beings were destined to reach for the stars. As much as it turned out to be an inevitability that intelligent life would emerge from the processes of abiogenesis, as well it is impossible to stop life from advancing to the point of exploring outside the bounds of its home planet.
By colonizing space we can harvest the vast natural resources available to us from the planets, moons and asteroids in the "local" vicinity but also in the further vastness of space. I do not mean to say we shouldn't thoroughly check the places we go to be absolutely sure we are not encroaching on any life. We must be absolutely sure to not repeat the mistake our species has made in subjugating less powerful populations. We have a moral duty even then to only take what we need and to take it responsibly.
We Like Doctors Without Borders, a non-profit organizaion called Engineers Without Borders does the same kind of work but helps improve the lives of people living in the third-world by reappropriating a device that extracts water from the air made by NASA used for space travel purposes. This illustrates just one example of how technological innovations that were created for one purpose can be of benefit to the people at large.
When the Japanese bombed Pearl Harbor, when the airliners hit the twin towers, there was no shortage of young men and women at the armed forces recruiting stations in the following days ready to give their lives and so much more in defense of their fellow citizens. The Federal Government rallied the populace to recycle metals, work hard, and conserve resources in the name of the "war effort" through various mass media means. This gave the populace a sense of unity through common purpose that unfortunately lacks today. Through this need to fight a common enemy people of disparate backgrounds are able to find common ground. This could lead to stronger communities and the social benefits that come from that.
We have dozens of corporations that rely for their sustenance on war or the threat of war. Several billions of dollars are spent annually on the maintenance of the usual branches of military by nearly every nation on Earth. We do need strong armies to protect us from not only the threats that terrorist organizations and nations run by dictators pose, but though I run the risk my sanity being put in question by saying this, we also need to be prepared for a military response, God forbid it should be required, to those that may harm us from outside our world.