I believe what the OP you replied to meant was that when entering your address in a form, the zip code allows for verification that the address was all entered correctly. Lets say someone wants to buy a book and they put their address in as 154 Broad Street, Philadelphia PA but they actually live in a neighboring county, when they put in their zip code the system will be able to see that the address was slightly incorrect and can be adjusted to ensure a smoother delivery.

I know where I live what "city" you live in can be a complex situation because of unusual classifications of cities compared to the rest of the country so I can see this being a useful system to have.