Rhetorical questions are often used as a metaphor for a question already asked. Examples may be found in the song Maria from the 1959 Rodgers and Hammerstein musical, The Sound of Music , in which the How do you solve a problem like Maria? is repeatedly answered with another question: How do you catch a cloud and pin it down? , How do you keep a wave upon the sand? and How do you hold a moonbeam in your hand? These responses may be taken as asserting that "the problem of Maria" cannot be solved; and furthermore the choice of cloud , wave and moonbeam as metaphors for Maria give insight into her character and the nature of the problem.
Dropping a rhetorical question into a persuasive argument is often a powerful form of persuasion. You present several facts and build up to a conclusion, drawing the conclusion out of the reader. For example, if you were trying to persuade the reader to support universal health care, you might ask “What kind of a country doesn’t ensure its citizens have access to health care?” For a reader to disagree with you, they would have to do some mental gymnastics in order to identify the underlying assumptions of the question–that universal health care is the only way to ensure all citizens have access to health care, or that if you disagree with the premise, you support an inferior version of the country.