Most passive sentences do not contain an agent; all active sentences contain an agent.
1. Passive voice is often used when the agent (the doer of an action; the subject of an active verb) is obvious, unknown, or unnecessary:
Oranges are grown in California.
Toyotas are made in Japan.
Her purse was stolen.
2. Passive voice is often used when the agent is known, but the speaker/writer doesn’t want to mention it:
She was given bad advice.
A mistake has been made.
3. Passive voice is often used when the agent is very general such as people or somebody.
English is spoken here.
The door should be locked.
4. Passive voice is often used when the speaker/writer wants to emphasize a result:
Several thousand people were killed by the earthquake.
5. Passive voice is often used when the speaker/writer wants to keep the same subject for two or more verbs but this would not be possible if both verbs were the same voice (active or passive).
For example, in a conversation about George, a speaker would probably use sentence a below rather than sentence b (both sentences are correct). a. George had several interviews before he was hired by a software company. b. George had several interviews before a software company hired him.