Rhetorical question is often used for comic effect, as in Shakespeare's Henry IV, Part 1 (1597) when Falstaff lies about fighting off eleven men single-handedly, then responds to the prince's doubts, "Art thou mad? Is not the truth the truth?"
On the other hand, Iago uses rhetorical question for sinister ends, persuading Othello that his loving wife is a whore. Iago hints with questions ("Honest, my lord?" "Is't possible, my lord?"), encouraging Othello to view his own unjustified suspicions as foregone conclusions.