When to use the Passive voice ????

Active voice is the better form to use. As a rule, the active voice is preferred for business writing, and for any other form of writing that requires the direct approach. The use of active voice increases vividness.

However, the passive voice is generally used when the subject of the sentence is indefinite, general, or unimportant. In the sentence, They mine coal in Pennysylvania, the subject is so indefinite that it is not clear what is meant by they. It might mean the miners, the people, or the companies.This sentence, and sentences like it, are improved by putting the verb in the passive voice.

They mine coal in Pennysylvania. (Poor)
Coal is mined in Pennysylvania. (Better)

They grow wheat in many of our states. (Poor)
Wheat is grown in many of our states. (Better)

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

The play, "Man and Superman," was written by Shaw. (Passive)
Shaw wrote the play "Man and Superman." (Active)

America was discovered by Columbus. (Passive)
Columbus discovered America. (Active)

In the first sentence, if you wish to emphasize the play more than the author, put the verb in the passive voice. In the third sentence , if you wish to emphasize the discovery more than the discoverer, put the verb in the passive voice.

The use of the passive voice is generally used when you want to emphasize the receiver rather than the doer. However, in the majority of cases the active voice is more effective than the passive voice.

Note - In general, one should avoid passive voice in GMAT, passive voice form may appear in a correct answer choice - especially in science, medical and technical writing styles.

The passive voice is required when the non - underlined part of the sentence contains the person or agent performing the action preceded by the word by.

e.g - The shuttle launch seen around the world by people of all ages, all races, and all religions.

This sentence is missing a verb, and it is therefore a fragment. Because the people who are seeing the launch are at the end of the sentence, preceded by the word by, we must use the passive voice to complete this sentence:

The shuttle launch WAS seen sround the world by people of all ages, all races, and all religions.

The link below also deals with usage of passive in GMAT sentence correction
Link to test magic forum - passive construction on GMAT