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.

Ah thank you - I get it now.

We wouldn't have that problem in France where the cities are clearly separated. The concept of "city" (commune) is the basis for the topography classification of the country since 1789.

So when I input an address, a suggestion system will give me several choices with several distinct cities and zips. If I input a zip code, I can get several choices of "places" under one zip.

A situation where your address is in the neighbour city should not happen (I put "should" because this is France :))