。
Where is the post office?
Literal
Post-office [topic-は] where [at-に] exist-[polite-ますか]?
どこにありますか uses the explicit existence-question form for finding a location. The に is required because ある takes locations with に. Slightly more formal than the shorter どこですか, because it explicitly asks 'where does it exist' rather than 'where is it.'