Why use passive voice?
Sometimes the agent is unknown, obvious, or unimportant. She was killed in the Mars invasion. What killed her? We might not know or care.
Sometimes we care more about the action than the agent. For example, London was evacuated before the invasion. Who evacuated it? Probably the proper authorities, but it doesn’t matter. The important thing is that Londoners got away safely.
Sometimes, as writers, we can use it to vary the sentence structure to put certain words at the end of the sentence and emphasize them. We can emphasize the agent, for example. Earth was invaded by evil Martians. We can put longer expressions at the end of the sentence to make it more readable. She was killed when Mars invaded Earth without warning anyone except London’s mayor.
In other words, sometimes you want to say something with more artfulness than active voice allows.
Next: How to change active to passive.
(Photo: NASA. London at night.)