。
Where's the post office?
Literal
Post-office [topic-は] where is-[polite-ですか]?
どこですか literally asks 'where is it' using the copula — grammatically simpler and more natural in casual-polite exchange than どこにありますか. The copula version glosses over the existence verb entirely, treating location like any other predicate, while the existence-question form makes the 'where does it exist' meaning explicit.