The other day I heard an interesting piece on NPR about the 50th birthday of the ZIP code. It's kinda strange to think about something as every-day as a ZIP code not having been around when my mom was born.
During the radio segment, they said that ZIP codes came out as a way to help sort mail more quickly by machine during an era when the US Post Office was booming. ZIP codes were introduced only a short time after Area Codes were, and it took folks a while to adjust to all these numbers. I can't even imagine being bothered by three or five new numbers when I have 20 million different online passwords that I have to remember.
I recommend that you listen to the story here, which includes the nifty jingle they came up with to help promote the new code. Also, check out these neat trivia tidbits and maps about ZIP codes.