If you say 'my favourite is Justin Bieber' you are simply stating that fact/information.
Both Timo and Jimmy talk about 'doubt' - I want to try to explain further.
If you say 'It would have to be Justin Bieber' you are communicating more. In fact you are implying that the answer is obvious, it must be Justin Bieber. Perhaps you are implying that he is clearly the best option and it would be foolish to choose anyone else! Or maybe it is obvious because the other person knows that you love Justin Bieber and you're his biggest fan. Another way to say this would be "Without a doubt it must be Justin Bieber."
By the way Justin Bieber is not a valid answer to the question "Who is you favourite band?" because he is not a band. He is a singer. 'A band' is a group of people not an individual.