Bassanio needs the money to finance a trip to Belmont, where Portia lives, so he can present himself in a way that will make her see him as a worthy suitor. He believes that if he is able to impress her with his wealth and charm, he will win her hand in marriage.
However, Antonio is facing financial difficulties and needs to borrow the money. This ultimately leads to the famous plot twist involving the bond with Shylock and the casket test, which forms the core of Shakespeare's play, *The Merchant of Venice*.