Who is benefited thereby? To whom is it a gain? The more usual meaning attached to the words is, What good will it do? For what good purpose? It was the question of Judge Cassius. (See Cicero: Pro Milone, 12, sec. 32.)
“Cato, that great and grave philosopher, did commonly demand, when any new project was propounded unto him, cui bono, what good will ensue in case the same is effected?” —Fuller: Worthies (The Design, i.).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894