Usually encourages the person or people being asked the question to reflect upon their own thoughts and beliefs rather than answer specifically out loud.
Speaker: "What defense to the homeless have if the government will not protect them?"
Guy: "Did you get some from her last night?"
Other Guy: "Is the sky blue?"
Office Worker: "How stupid is this new filing system we have?"
"I thought you loved children?!"
"I do! I just wanted to make sure you remembered.(:"
"You're a crazy hoe, I'm out! Call me when you need child support. Better yet, I'll give you a booty call within the week."
Now you see, the moral of the story is never to ask rhetorical questions.
It typically cannot be answered since it would have previously been answered.
Therefore, the expectation of that person or persons being questioned is that they need not be expected to respond to it (wastes time and useful energy).
The questioning tends to be done somewhat annoyingly to add emphasis to an initial query. However, the net result is more like an annoying interrogation.
See also: retorical
"Yes I did. I searched thoroughly but without success."
"Are you sure?" He asks incredulously.
"I believe I've answered your question already. That was a rhetorical question.!"
I ask myself, "If my ability to find something so simple and familiar is so sadly doubtful why the f... don't you look for it yourself?"
Pissed off with this moron, I cut discussion and exit the room.