The UK's Post Codes are more than just alphanumeric, they're deliberately designed to make confusables reudndant. So e.g. if SO1 5LI existed, a dedicated postal OCR (or an experienced postal worker) knows that's SO1 5LI it can't be 50I SL1 or whatever such confusions never exist.

Historically they were directly functional, the first part (it doesn't matter to a machine how the code is written, but they notionally have an "Out" section first and an "In" section) SO1 told sorters which local postal sorting office to send this to, SO1 would be in Southampton, a port city on the south coast because the first two letters are somewhat mnemonic.

But today actually a machine translates post codes into numeric codes that aren't used by the general public (although they are published, and you can mechanically annotate bulk mail with the newer numeric codes for a discount). This is fortunate because once the codes were broadly accepted and used, they became a matter of vanity, like other aspects of addressing in the UK (and presumably other countries)

For example, I grew up in Prestwood, an unimportant dormitory village (ie most of the adult population work in London, and just come back to Prestwood to sleep) in Metroland. But the nicest houses in Prestwood didn't consider themselves to be "in Prestwood" at all, they'd argue that since they're on a hillside near the more notable and expensive town of Great Missenden, they're actually living in Great Missenden. And they'd insist their post codes should match this er, Truth.

So making the post codes merely a human artefact, and the real coding is numbers humans are normally unaware of, allows a house that is clearly in Prestwood to instead have an address in Great Missenden, complete with Post Code, yet the sorting people can give it to a guy whose job is to deliver to Prestwood, because that's where it actually is.

(One downside to the sort of people who live in a dormitory village, beyond the fact that they aren't really around most of the time and so don't care about lack of amenities, is that they see themselves as important and powerful and so they might waste a lot of taxpayer money demanding that some minor bureaucrat "fix" their address and it's just easier to say OK than fight them. There are upsides though, they have plenty of cash, so you get pretty good loot on Halloween and decent donations to charities)

Anyway, back to the HN relevant point, there are commercial products which can go from a post code to a list of "delivery points" (addresses, but the postal delivery people only really care about where the post is delivered to, so from their point of view that's what we're encoding) and that can be turned into a drop-down, or it can be auto-completed from the house number if that works, or whatever you customise. This is a place where you can do a poor job, produce a working product but maybe cut your purchase completion rate slightly, because your address entry is annoying and customers walk away. Or you can do a really slick job, but miss one edge edge case and find oops, buying your product from one town in Yorkshire was virtually impossible due to a bug (there's usually a manual entry option, but obviously normal users hate that and won't bother)