Safe Haven. If we nuke Earth, or a natural catastrophe occurs (extreme impact or geological activity).
Spark Important Advances. Requires tremendous scientific and technological development, fuel economies, foster international cooperation.
Future Challenges. Humans have always been explorers. With most of the Earth covered, where are the future challenges for our children explore?
Expansion Room. Even with drastic population control, it is during your grand children's youth that the population will have doubled.
Gives DoD something safe to do. To do this will require enormous government investment, possibly pulling money from the Defense Department.
Sustainability. Terraforming, initially with enormous greenhouses, means those settled will have a stable, sustainable environment, food and fuel resources.