You say "brand new" when you want to emphasise that it is new. Instead of saying, say, "really new" or "very new" as you would with most adjectives, we say "brand new".

It's colloquial, but brand new is more recent than new. "We have some new items this month, and these ones are brand new today."

The iPhone 6 is brand new. The newest if the new.

We say 'brand new' to emphasise the newness of something. 'I can't believe this bag's broken already. It's brand new!' 'Brand new' is a common collocation, almost like a kind of set phrase. We use it in the same way as 'boiling hot' or 'freezing cold', simply to add emphasis to the adjective.

"brand new" is newer