The backup is not necessarily against a man-made catastrophe. A stray asteroid in the right place at the wrong time is perfectly capable of fucking up Earth without our assistance.
Surviving even a Cretaceous–Tertiary sized extinction event on Earth would be a hell of a lot easier than trying to terraform and permanently colonize a planet like Mars that is missing its magnetic shield.
In a Lagrange point it's actually quite trivial to create a man made magnetic field to protect the planet. Dr Jim Green, director of planetary science at NASA worked on it and fleshed out the details, it's viable and would actually restore a lot of the atmosphere stripped away constantly. As far as colonisation problems go the magnetic field is far down the list. There's a lot of work being done in this field.
