Think of it as a mitigation. If the earth took a hit from a serious comet, what would happen to humanity? Learning to populate and possibly eventually "terraform" Mars is one heck of a project. If Musk wants to spend his money that way, who's to argue? We all learn a lot along the way.
Mars cannot be terraformed. It’s too small to hold an atmosphere. Even if we collected the entire asteroid belt and dragged Pluto to Mars, we would only increase its mass by about 5%, which is still too small.
The best we will be able to do is domed cities.
Venus is a better candidate, if we can figure out how to relieve the greenhouse effect.