There's also an interactivity requirement (as in, this is impossible without interactivity) and an interactivity desire (as in, UX folks want an interactive feature).

So many websites now pretend they're in the former category, but would work just fine if they implemented a less-interactive form. That's how it was done in the old days! "No, you can't have that. Here's the best we can do." "Oh, that's fine."

But I expect more of what's driving complicated backend/frontend split design these days is shipping the org chart (in companies large enough to have frontend and backend teams) and technology-by-consultant (where the right technology is whatever you can get a consultant in).