Was it really necessary for Portia to go to Venice?

No, her trip to Venice wasn't necessary, but it was definitely productive. By dressing as a man and having the unique chance to prove her intelligence, Portia is allowed to help Lorenzo, his closest friend, and independently prove her worth. 

Portia’s journey to Venice was unnecessary.By doing so, and impersonating as a lawyer she was able to manipulate the contract and save Antonio, saying that Shylock could cut a pound of flesh but could not spill a drop of blood as it was not mentioned in the contract. If one drop of Christian blood was shed all his property would be confiscate by the state of Venice under Venetian law.