Rhetorical Question “Who knows?” “Did you hear me?” “Ok?”

The definition
It a special syntactical stylistic device the essence of which consists in reshaping the grammatical meaning of the interrogative sentence.
There is an interplay of two structural meanings: 1) that of the question; 2) that of the statement (either affirmative or negative).

Some examples
Are these the remedies for a starving and desperate populace? (Byron)
If you prick us, do we not bleed?
If you tickle us, do we not laugh?
If you poison us, do we not die?
And if you wrong us, shall we not revenge?
(Shakespeare)

— Prof. Popov
«…the rhetorical question is equal to a categorical pronouncement plus an exclamation».

...Shall the sons of Chimary
Who never forgive the fault of a friend
Bid an enemy live?...”
(Byron)
Some more examples

Did not the Italian Mosico Cazzani
Sing at my heart six months at least in vain?
(Byron)
Have I not had to wrestle with my lot?
Have I not suffered things to be forgiven?
(Byron)

The stylistic effect of the transference of grammatical meaning can only be achieved if there is a simultaneous realisation of the two meanings: direct and transferred.
Both the question-meaning and the statement-meaning are materialised with an emotional charge, the weight of which can be judged by the intonation of the speaker.

Is the poor privilege to turn the key
Upon the captive, freedom?
(Byron)
Functions and meanings

“Is there not blood enough upon your penal code, that more must be poured forth to ascend to Heaven and testify against you?”
(Byron)
Who is here so vile that will not love his country?
(Shakespear)
Functions and meanings

So rhetorical questions may also be defined as utterances in the form of questions which pronounce judgements and also express various kinds of modal shades of meaning, as doubt, challenge, scorn, irony and so on.
Rhetorical questions, due to their power of expressing a variety of modal shades of meaning, are most often used in publicistic style and particularly in oratory, where the rousing of emotions is the effect generally aimed at.