People do things like include two suburbs because they know that (eg) in order for it to reach their address correctly, it must first reach the human at point A that will correctly pass it to the human at point B, where it otherwise cannot arrive at point B because point B does not receive normal postal service, possibly because of a jurisdiction dispute in 1974 that placed their address in a zip code that’s different from all of their neighbors.

Can you tell that I know a bunch of people with these kinds of issues? You may already know that programmers mess up names all the time, telling people that their last name must be their surname, or that you can’t be an O’Reilly or a Robertson-Peele, or that “Mary Anne” is not a single name, or that your middle name cannot be your “primary” name, or that you must have more than one name, or that your legal name is invalid because it’s not the name you were born with, or that all 3yos have names, etc.

Well, take all of those issues, and add the vagaries of geography, and you’ve got mail delivery.

I have a weird address, and easily 30% of websites insist on fucking it up by applying validation rules that might make sense from 20,000 feet, but don't actually work in practice for our address.

The most straightforward of them is that some validation services insist that our ZIP code is for the next town over instead of the one we live in, which has its own post office. Nothing correct happens if our mail goes to the wrong post office because they (rightly) have no idea how to deliver mail to us.

I wouldn't be so confident that 100% of that 70% don't know their own address. For at least some of those cases, I'm willing to bet they know something you don't about the vagaries of mail and package delivery to their address.

Modern postal OCR is generally good enough to detect bad addresses, but my point was people still beat it when the domain is so constrained.