A question asked without expecting an answer, either because the answer is already implied, or because what matters is the doubt it exposes.
In Madame Bovary
Why not end it all? Who would stop her?
"Why not end it all? Who would stop her?" Emma does not expect an answer, because the questions already carry it: nothing and no one stops her. The interrogative form lets the idea of death appear as a cold possibility, almost as freedom.