What is the purpose of the "+4" in ZIP codes?

according to wikipedia:

ZIP+4

In 1983, the U.S. Postal Service began using an expanded ZIP code system called "ZIP+4", often called "plus-four codes", "add-on codes" or "add ons."

A ZIP+4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. Initial attempts to promote universal use of the new format met with public resistance,[citation needed] and today the plus-four code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 code from the address and—along with the even more specific delivery point—sprays a Postnet barcode on the face of the mailpiece that corresponds to 9 digits.

For post-office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often one of the following: the last four digits of the box number (e.g., PO Box 58001, Washington DC 20037-8001), zero plus the last three digits of the box number (e.g., PO Box 12344, Chicago IL 60612-0344), or, if the box number consists of fewer than four digits, enough zeros prepended to the front of the box number to produce a four-digit number (e.g., PO Box 52, Garrett Park MD 20896-0052). However, there is no uniform rule, so the ZIP+4 code must be looked up individually for each box.

It is common to use add-on code 9998 for mail addressed to the postmaster (to which requests for pictorial cancellations are usually addressed), 9999 for general delivery and other high-numbered add-on codes for business reply mail.[citation needed] For a unique ZIP code (explained below), the add-on code is typically 0001.