The passive voice is used for:
mistery
legal documents
to leave the subject out of a sentence or to put the emphasis on the object

The rules are:
1. There is no subject or
2. If there is a subject, the emphasis is not on it, but on the object
3. The use of the reflexive (there is no subject, and it's conjugated in the third person)
4. The use of ser and/or haber
5. The use of the past participle

Examples:
1. The door was opened (La puerta fue abierta). We don't know who opened the door. There is no subject for the action to open.
2. I was hired in September. (Fui contratada en septiembre). We don't know who hired me. There is no subject.
3. Cars washed here. (Se lavan carros aqui) We don't know who is washing the cars. There is no subject.