When to use the passive voice

As a rule, the active voice is preffered for business writing, and for any other form of writing that requires the direct approach. The passive voice expresses reserved action, since the receiver comes before doer.
The passive voice is generally used when the subject of the sentence is indefenite, general or unimportant.

They mine iron in Germany. (poor)
Iron is mined in Germany. (better)

The passive voice is also used when what was done is more important than the doer of the action.

The play, "Man and Superman," was written by Shaw.
Shaw wrote the play "Man and Superman".

In the first sentence, if you wish emphasize the play more than the auther, put the verb in the passive voice.