I did expect new areas to open up when I built it but no such luck. That is why I was wondering if I missed something
You probably already "built" the soil enhancement, habitat improvement, and terraforming tiles on your planets before the orbital terraformer completed-particularly as auto-terraforming is on by default (you can change this via the planetary details screen, along with auto-upgrade of improvements).
You should certainly notice it in the future, though-try colonizing a class 4 planet and watch what happens to it.
