I assume you're asking this question in good faith. A mailing address alone is often ambiguous, eg tons of duplicate street names, people write the wrong city, and typos and missing unit numbers are common. The ZIP code doubles as a sanity check against the rest of the address. USPS will usually figure it out without one, but it takes extra time and effort. Also, a person's zip code also doubles as card verification and sales tax. Maybe the opposite question should be asked, why do we need addresses? Your residence could be defined expanding on its zip code.