As defined in our glossary, a question is "rhetorical" if it is asked merely for effect with no answer expected. The purpose of this figure of speech is not to secure a response but to assert or deny a point implicitly. A rhetorical question may serve as a subtle way of insinuating an idea that might be challenged by an audience if asserted directly.
The following passage from Richard Russo's novel Straight Man (Vintage, 1997) contains two rhetorical questions. The narrator is William Henry Devereaux, Jr., chair of a college English department, reporting on a telephone conversation with his mother.
A couple days after she'd begun the task, she called me, all excited, to say that she'd discovered two hundred pages of a novel in manuscript, dating back nearly twenty-five years. "Isn't it amazing?" she wanted to know, and I didn't have the heart to tell her that it would have been more amazing if there hadn't been two hundred pages of a novel. He was an English professor. What did she expect?
A different sort of rhetorical question is hypophora, in which a speaker raises a question and then immediately answers it. During his tenure as Secretary of Defense, Donald Rumsfeld frequently employed this strategy when addressing the press. Here's an example from a news briefing on October 26, 2006:
You say have they agreed to "it"? Are they meeting and having discussions on these things? Yes. Have they been meeting for some weeks and months? Yes. Does that imply a certain amount of understanding that that process might be useful? Yes. But can I say that they--that is to say the prime minister and his government--have come down and said, yes, we'll do this, we won't do that or, yes, we will do this, we won't do that, and we'll do it by this time? No. I--one would have thought they might have announced that if they decided all of that.
Hypophora, like a conventional rhetorical question, enables a speaker to control a discussion and shape the terms of an argument.