When is passive voice used?

When the subject is unknown or not important

For example: The man was killed last night.

(I don't know who killed the man.)

When the identity of the subject is clear form the context

For example: A meeting will be held on Friday.

(We can assume that the boss will hold it.)

When the subject is intentionally not to mentioned

 For example: The secret was told in the party.

(I know who told the secret, but I want to keep it as a secret.)