ABSTRACT
The passive is used when the writer wants to focus on the result, not on the cause:
The company was founded in 1925 by Walter Trimble (passive)
Walter Trimble founded the company in 1925 (active)
In the first sentence, the emphasis is on the company, in the second on Trimble. So the passive is often used in written English when the cause (a person or thing) is less important or unknown.
