I believe the demand for this was to visualize attributes on a map by coloring polygons, and the dataset was addresses that already had zip codes.
(Please stop coloring polygons by attribute: it gives false impressions because areas are unequal. Instead, use a scale of dot density. Or marker size, which has its own perceptual issues.)
However misguided, coloring zip code polygons gives you things like maps of customer data. But you could just as well geocode each address and assign it to a county or other boundary. (news flash: the geocoding database is derived from the same line data as the ZIP code project.)
Some zip codes represent a single building. Example 20500 is the White House. The Pentagon is so big it has several zip codes.
https://convene.com/catalyst/office/buildings-new-york-city-...
It used to be that the first 3 digits of a zip code sent that mail to the "main" post office building (the one that has loading docks for like 10-30 tractor trailers on one side of the building). Even if you were mailing a letter to the same zip, it went to that building to be sorted and returned to the local office before delivery. Nowadays, everything goes to central facilities where stuff is scanned and sorted. In Kentucky, almost all mail goes through Louisville.
The "city" on my address growing up was a CDP[1], and it is now a city that I live 2 miles from rather than the one that I live less than a mile from.
And in no instances have I ever run across use cases where these are worth paying any substantial money for. If USPS is going to jump into the fray here, really bad look (and probably net negative revenue for them) to offer as a paid product.
Whatever you're grouping is likely better served by some grouping based on geography, not based on delivery routes. You probably wouldn't group by telephone company central office either, unless you're telco related.
We want to group by geography, zip codes are easy and vaguely geographic, lets use that is kind of a valid decision, but it's deeply frustrating :p
In fact, these were invented by Nielsen, and they determined them just by measuring which city's TV stations are predominantly watched in each county, and assigning each county to one DMA based on that.
The ATMs never got mail.