Tubal's appearance in Act III, Scene 1 of The Merchant of Venice juxtaposes the preceding conversation between Shylock, Salarino, and Salanio in which Shylock expresses his feelings about the objectively horrible treatment he has received from Antonio. During Shylock's conversation with Tubal, Shylock reveals the depth of his desire for revenge for Antonio's treatment and the depth of his outrage that his daughter robbed him in order to abandon for a Christian. Without Tubal's presence in Act III, there would be no revelation of the powerful motivation that drives Shylock to proceed with his grissly court suit against Antonio, a suit in which he wishes to exact revenge for Antonio's offenses and for representationally for Jessica's offenses.
In the play 'The Merchant of Venice', Act III, Scene 1 Tubal's appearance was important because he was there to inform Shylock that in Genoa he have heard about Jessica, Shylock's daughter but was unable to find her and the news of antonio's ships which was lost while returning from Tripolis. All the informations given by tubal about jessica made him angry and the information about Antonio's bankruptacy made him happier ever to take revenge against Antonio for the insult he did to shylock in the Rialto.