Some zip codes cross state lines. If you need to know what state somebody is in you can’t get it 100% of the time from state. In practice that only affects a few.
Towns will be a much bigger problem. Lots of zip codes cover more than one small town. If you have a common street name like Main St, Church St, Park St or Railroad St you’d better include the town name. If zip code 12345 covered 3 small towns they might all have a Main St.
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