What is the use of a door?

http://www.tepco.co.jp/en/press/corp-com/release/2013/1224790_5130.html

February 15 , 2013

-At 10:50 AM on February 15, while carrying goods out of the large carry-in entrance in Unit 6 Reactor Building with its inside door (the door which opens by lifting up) open and its outside door closed, the inside door fell. The door which fell did not fall over. No injury has been reported due to the incident and the incident has not affected the plant condition. The condition of the inside door and the cause of its falling are currently being investigated.