In 1943, before the ZIP Code made its first appearance, the US Postal Service began using one and two-digit numbers to sort mail. These numbers designated the city and the state where each mailpiece was to be delivered. At this time, the US Postal Service was distributing approximtely 20 million pieces of mail per year.
With the growing population in the 1960s, an increasing need of the ZIP Code was realized. On July 1, 1963, the internal advisory board recommended the Post Office introduce the Zone Improvement Plan Code. This code divided the entire country into coded delivery areas.
With the introduction of this new system, people became concerned that they were being turned into numbers. To help alleviate some of these concerns, the US Postal Service created a marketing campaign around a character named Mr. ZIP. Mr. ZIP could be seen in TV advertisements as well as magazine advertisements.
By 1969, it was clear that the marketing campaign proved to be a huge success, with 83% of Americans using the ZIP Code on their mailings. In 1983, the ZIP Code was expanded to nine digits, which told the US Postal Service which side of the street mail was being delivered to.
Today, ZIP Codes are used in 31-digit barcodes that tell the US Postal Service a whole host of different things such as whether the mail was presorted, whether it is first-class or a periodical, and which business sent the mail. Today's ZIP Codes can help the US Postal Service track nearly all 160 billion mailpieces being sent around the country every year.
Source: http://nation.time.com/2013/07/01/the-zip-Code-turns-50/