I need to explain very clearly to an upper intermediate student the difference between It and That. Eg. You are wearing a t-shirt with The Beatles and student wants to say 'Oh, that's my favourite band' or 'It's my favourite band'. I would say 'that's' but I can't find a good enough reason for my choice - if there's one. Any suggestions why we should use one or the other, would be extremely welcome.
A good question, but in your example I would explain that the speaker would say "That's my favorite band" if there were some visual or audible stimulus that provoked the remark such as in your example with the T-shirt picture OR the speaker hears a Beatles song in a mall or shop without any previous discussion about the Beatles. It's my favorite band might be used when there is some discussion occurring about the Beatles, during which the speaker makes the statement.
Again, a good question and I'm sure there might be other possibilities, but this is what occurred to me.