Terraform can make all of the changes to your environment very quickly. No longer do you have to wait for days whilst someone follows the run book by hand. A change is made to the code, merged, and Terraform instantly updates every environment simultaneously to include the new change.

As your infrastructure is now defined in code, you can check it into source control. This means that you can change your code, roll it into an environment using Terraform, and try it out. If the change is not good, you can go back to the previous version of the source control and rerun Terraform. Then Terraform handles restoring the environment back to how it was. If the change is good, you can check that into source control and roll it into your other environments.