Reasons for using the passive voice

• To promote a sense of scientific objectivity

  Passive sentences are commonly used in scientific writing where the emphasis is on the action— the test or experiment—and not on the scientist who conducts it:

In the first test, the APC shell was fired from 100 metres at an RHA target.

Someone is doing the firing here, but it is not important to the writer/researcher's
purposes to identify the actor.

• When the recipient of the action is more important than the actor

  The passive voice is most useful when you want to emphasize the outcome or consequences of an action:

The UN vehicle carrying General Dallaire was fired at by a Rwandan para-commando unit.

Creating a passive sentence here rightfully puts the emphasis on General Daillaire's brush with death at a roadside ambush and away from the perpetrators, who are less important to the point of the sentence.

• If you cannot be sure who is responsible for an action

  Obviously, if the doer or actor is unknown, then a passive sentence that emphasizes the action is the only choice:

A box of hand grenades was stolen from the supply depot last night.

• To evade assigning responsibility for an action

  A rather dishonest use of the passive voice is all too common in documents and statements produced by government bureaucrats or in reports on actions and decisions taken by committees and organizations:

Many decisions were made concerning the NATO targets.

The use of the passive here avoids assigning responsibility for the decisions. Writing like this is always suspect; it gives the impression that the writer has something to hide. A direct, active sentence makes the point clear and satisfies the reader:

The Commander-in-Chief made the crucial decisions concerning the
NATO targets.

• When you wish to avoid being too personal

  If you have to write a memo to someone concerning a disciplinary action or a matter of proper behaviour or procedure that has not been followed, your point will be taken as less of a personal attack by the individual addressed if you choose the passive voice over the active. Compare the following:

If you make a mistake like this again, you will have to face the consequences.
(active sentence)

A mistake like this must not happen again or there will be consequences.
(passive sentence)

Use of the passive here is less abrasive, less affronting to the individual addressed. For reasons of politeness and civility the passive voice may be preferred.

• To avoid overusing the personal pronoun "I"

  For stylistic reasons, occasional passive sentences can act to relieve the regular rhythm of the S-V-O sentence. This is especially appropriate in a piece of writing in which the author is the main character and many sentences begin with the first-person pronoun "I."

I conducted the operation with the full approval of NDHQ.
(active)

The operation was conducted with the full approval of NDHQ.
(passive)

Writers sometimes formulate a sentence in the passive voice for the sake of sentence variety, but this strategy should be used sparingly. And that, apart from a prescriptive use of the passive in scientific writing, is the best advice.