Shakespeare relies mainly upon his two setting in Merchant of Venice to demonstrate the conflict and contrast between Romanticism and Pragmatism. Belmont is the fairy-tale island, isolated from the hum-drum of every day business dealings. It includes riddles and marriage and playfulness. In contrast, Venice exemplifies the tedious business dealings in life and the fulfillment of the letter of the law (in the Duke's court). In Venice, daughters betray their fathers; minorities are insulted; revenge is sought; and "justice" is enacted (without genuine mercy). In Belmont, loose ends are tied up, new marriages move past early misunderstandings, and the truth is revealed.
Another contrast is, of course, the clash between Shylock's worldview and the Venetians. Shakespeare uses this conflict to shed light on the inequality of justice for certain groups of people versus the elite.