Because the address environment is so diverse in the US, and must identity locations like the end of a dirt road in Kentucky, a small island in Maine, and a high rise in Manhattan and a suburb in Takoma, the zip + 4 provides differing information. In a sub-division in Kansas City, the last two digits will identify two houses, yours and your neighbor across the street. In a Post Office box environment it will be a section of boxes. The USPS determines the necessary delivery information for the specific address and assigns a zipcode and uploads it to the national database.
These codes enable automating sorting of the mail nationally. Without them the USPS could not sort the hundreds of billions of pieces of mail done each year. It works like this. When you drop your letter in the mailbox in the UK it is flown to a large Airport Mail Facility in the US. It will be processed on a machine with optical character and bar code readers that will send it into one of several product lines depending on the preparation you have done. It will then be handled according to your preparation through automated systems, receiving in the process a zip+4 barcode. Even if your mailpiece is handwritten and has no zip code it is unlikely (25%) a person will engage it until it reaches the hand of the letter carrier or box clerk. If it is typewritten the chances are much smaller, add a zip code and then a plus four and you get down to less than 7%.
. Hope this helps. Fortunato