The door was opened expresses an action. It's passive voice. That is, whatever or whoever opened the door is not important. What's important is that the door underwent an action. The passive voice is used when
(a) you don't know who the agent is,
Ex: The Universe was created (by ?).
Ex: That building was erected in 1993 (by ?).
Ex: The project was completed (by ?)
(b) you know who the agent is but you would rather not say, because if you did say who it was that information could get you into trouble.
Ex: The last piece of cake was eaten this morning (by me).
Ex: My homework was done yesterday (by my mother).
c) You know who the agent is but that information isn't important.
Ex: The door was opened last night (by me. Everyone knows I always open it.)
All the best.
