Not only are their PO Box only zip codes, but there are also zip codes that are associated with various business and government entities. In those cases the USPS literally doesn't care what you put for the address bc they will hand over all the mail addressed to that zip code to the entity and let them handle it.
The most famous example of this is the zip code 12345, which is actually for GE in Schenectady, NY.
The IRS also has some of these dedicated 5-digit ZIP codes. For example, 00501 is the IRS processing center in Holtsville, NY. (According to Wikipedia, this is also the lowest-number ZIP code currently in use in the US.)
