There's not much we can do if Earth is hit with another giant asteroid. Or some massive biological or radiological disaster. Or some other type of impossible to predict or mitigate disasters that could render Earth uninhabitable for humans. Musk's goal is to make the human race multi-planetary, not escape to Mars. Just as you should back up your critical data more than one hard drive, or run redundant servers to ensure your service runs reliably, populating multiple planets would ensure the continuity of the human race should some disaster befall us on Earth.