History of the United States Postal Service [usps.com...]
ZIP+4 - Introduced in 1983, the ZIP+4 code added a hyphen and four digits to the existing five-digit ZIP Code. The first five numbers continued to identify an area of the country and delivery office to which mail is directed. The sixth and seventh numbers denote a delivery sector, which may be several blocks, a group of streets, a group of post office boxes, several office buildings, a single high-rise office building, a large apartment building, or a small geographic area. The last two numbers denote a delivery segment, which might be one floor of an office building, one side of a street between intersecting streets, specific departments in a firm, or a group of post office boxes.
Keep in mind our ZIP+4 system has been in place now for 25 years! It's been pretty much transparent to the residential customer. But, when it comes to businesses, that +4 will determine the efficiency of deliverability.
I used to do quite a few commercial mailings. The savings by providing a list that was sorted to the nth degree could equate to quite a large sum when mailing 500,000 pieces. There could be a savings upwards of $.05 per piece!
So, ZIP+4 is important for business use. The U.S. Post Office have implemented technology that makes it transparent to the residential customer unless of course you look at all of your mail. The USPS is going to spray your ZIP+4 on every piece, or almost every piece. ;)