why do we need to use city and state in our addresses if ZIP codes are already linked to the city and state?

Question

why do we need to use city and state in our addresses if ZIP codes are already linked to the city and state?

In: 100

Answer 1

From other comments it seems zip codes are mapped out different to UK post codes, but over here I can put just the house number and post code (37, AL9 4QA) and the mail will be delivered.