A question asked without expecting an answer, because the answer is already implied, or because what matters is the doubt it exposes.
In Macbeth
Will all great Neptune's ocean wash this blood / Clean from my hand? No, this my hand will rather / The multitudinous seas incarnadine, / Making the green one red.
The speech opens with "Will all the ocean of great Neptune ever wash this blood clean," and he himself answers, "No, rather." The question already carries its answer, and what matters is the certainty that the stain will not come out.