A pressure test is a safety precaution.
The city requires that your natural gas line hold 15 (psig)pounds per square inch gauge of air for 15 minutes. This is to ensure that a hazard to persons or property does not exist.
Natural gas lines that have been off for over a year or have been determined to have questionable integrity are subject to a pressure test. Additionally, all new services must pass a pressure test before being put into service.