Questions & Answers about Скажи мне, зачем тебе новая куртка?
Зачем literally means “for what purpose” or “why (in order to…).” It asks about the goal or purpose behind an action. Почему means “why” in the sense of a cause or reason.
Example:
– Зачем ты купил книгу? (“What’s the purpose of buying the book?”)
– Почему ты купил книгу? (“What’s the reason you bought the book?”)
In the full version of the underlying clause, Russian uses the structure мне нужна книга (“I need a book”), where мне is dative. Our sentence omits нужна, but keeps the same syntax:
– Скажи мне, зачем тебе новая куртка (нужна)?
So тебе remains dative because it marks the person who needs something.