Interior access is needed to ensure the dryer vent system is functioning properly. The difference between cleaning with interior access versus cleaning with only exterior access only is significant. The goal is to ensure that the dryer vent is working properly. A properly functioning dryer vent system has good airflow from the dryer to the vent termination or building exterior.
While an exterior cleaning seems simple and easy it has a few deficiencies and having access to the interior of the unit is the only way to ensure a job well done.