This article is an FAQ with a tutorial on how to open the vent door.
The benefit of opening the vent door is to allow fresh air to enter the room. This lever is located near the universal power cord outlet. However, most commonly this vent door is not recommended to unscrew. This causes outside air to leak in while running the unit during heating or cooling. As a result, this would cause your unit to continuously operate and be unable to reach your set point. All units come with the vent door shut and screwed in.