[masterso]

We do have to spread out to other place if we are to survive as a species, that much is absolutely sure.
There are all manner of catastrophic physical events that can, and with almost complete certainty, do massive damage to the Earth. Top of my list of things to worry about is a large asteroid strike. There's ample evidence in the past of huge strikes that have caused massive damage and absolutely no reason why it couldn't happen again any time soon. And we have no way of stopping it happening.
Also up near the top of the list are such things as massive volcanisation that would pump enough crap into the atmosphere so as to make life very difficult for everything alive. Also some nasty virus that has a long incubation period, spreads by air, has a very short active period, and is fatal; There's some unpleasant people out there that could, in theory, design such a thing.
The only way to ensure we don't lose all our eggs in the one basket is to get out of that one basket. And if you want to take the greenie point of view if we reduce the human load on this planet if will do just fine (much better in fact) without us burning everything that will burn and pumping crap back into it.

The "remove all the eggs from one basket" thing seems, to me at least, to be the most sensible argument in favor of off world colonisation. Equally we could develop technologies to stop or limit the effects of "extinction level events" (asteroid impacts or volcanic eruptions, etc..).