Why did Shakespeare insert the "lottery of the chests" tale into The Merchant of Venice?
We know that the subplot of the three suitors choosing among the gold, silver, and leaden chests was taken by Shakespeare from source material that was entirely separate from the main "pound-of-flesh" story. The "lottery of the caskets" is an ancient tale that stands on its own, and so the question becomes why did Shakespeare insert this sub-plot into his play? We note that the "correct" choice, as made by Bassanio, is the lead casket with its legend that he selects this chest "must give and hazard all he hath" and an image of Portia inside. We note further that the sub-plot is fully resolved by the mid-point of the play. Bassanio's choice reflects favorably upon him as a man who appreciates humble self-sacrifice. As such, it greatly improves our estimation of Bassanio who has henceforth enacted the role of a wealth-seeking suitor. But more importantly, the values attached to the leaden casket are Christian values that appertain to the "hero" of A Merchant of Venice, Portia. The sub-plot allows for the exposition of Christian ideals that Portia subsequently expresses in her self-less defense of Antonio (a man whom she knows only through Bassanio's word), most especially in her "quality of mercy" speech.