The real meaning is, "Who benefits?" or "Who stands to gain?" or "Who gains by it?" or "To whom is it an advantage?" This phrase is often but erroneously translated "For what good?" or "What good will it do?"

Cicero (pro Milone 32) wrote about a Lucius Cassius (a tribune of the people in 137 B.C.), as a quaesitor judicii [judge] who presided in a trial for murder and who advised the judices [jurors] to inquire, when there was a doubt as to the guilty party, who had a motive for the crime, who would gain by the death; in other words, cui bono fuerit? [Who might have gained or benefited?; because fuerit is the perfect subjunctive mood form of the verb "to be"].