In Part 1 of this series, I listed the first instance when using a passive sentence is more effective than using an active sentence (when the doer is not important). The next instance, although similar to the first, takes it one step further.
The Action is More Important than the Doer
When the doer is important to the idea of the sentence—but you want the emphasis to be on the action—use a passive sentence. The difference is subtle, but there:
Divers have explored over 3,000 caves worldwide.
Over 3,000 caves worldwide have been explored by divers.
Notice how the reader's attention is naturally more focused on the subject of the sentence—in the first example, on "divers," and in the second example, on "caves." I, the author, want readers to focus on how many caves have been explored rather than on the fact that divers did it. But unlike the example in Part 1, the doer is important because it clarifies that I'm talking about underwater caves versus dry caves.
Other Examples of This:
The coffee was prepared by the wives before being drank by the husbands. (The coffee is what I want readers to focus on, but the doers still matter.)
Peace can be had by those who follow Christ. (The emphasis is on the peace to be had, but who the "havers" are matters as well.)
Wars can be fought and won by words and diplomacy. (The emphasis is on the wars, but how they're won is still important to the meaning of the sentence.)