The scene where Portia's suitors choose from three caskets to find out if they qualify for marrying Portia is another theatrical thing in the Merchant of Venice. Prior to this scene, Portia mocks all her possible fiancés except for Bassanio in the conversation with Nerissa, and it is clear to the audience that Portia will reject a propose by anybody but Bassanio. And indeed, Bassanio is the one who is chosen as the fiancé of Portia. However, Shakespeare chooses to make Bassanio the fiance through this unique idea of three caskets rather than having Portia simply choose him.
One of the functions of this scene is that it allows the audience to see a clear distinction of the ideologies of the three suitors. The reasons that each suitor gives before he chooses a casket reflect the philosophies and ideologies of the suitors. Through this scene, one can see that there is a clear contrast between the philosophy of Bassanio and the other two suitors.
Secondly, instead of having Portia choose her fiancé and explain to each suitor why or why not he was chosen, Shakespeare makes each suitor choose his own fate and the casket explain why or why not he was chosen. This makes the reasons given for the rejection or the acceptation of the suitor appear to be much more credible than being told through Portia's mouth. It is also important that the phrases in the caskets appear like moral precepts. It gives almost an absolute justice to the judgment by the caskets.
Finally, the casket system allows every person, regardless of class, to be judged equally. The caskets function as dogma that judge people based solely on their philosophies and morals.
This whole scene allows Shakespeare to articulate the problem with the nobles that were criticized by Portia in the prior scene. Throughout the play, the nobles are portrayed as superficial people who care only about their appearance, as opposed to Bassanio, who chooses the lead box because he declares that appearance shows the least of something. And because the caskets judged Bassanio as the right one, the audience is directed towards receiving Bassanio's idea as an absolute truth. What is implied by Bassanio's statement is that the morality of a person is higher than his class or appearance. The fact that the suitors were all judged equally under the casket also emphasizes this idea, as the suitors are judged stripped off of their social status and only by their morals or ideologies.