Modern zoning emerged largely as a public-health response to nineteenth-century cities where a tannery could open next to a nursery with no legal obstacle. The underlying insight wasn't really about noise or smell; it was that some uses degrade the value of adjacent uses, and left alone, the market has no built-in reason to prevent that.
Mixed-use done deliberately
Good zoning isn't strict separation for its own sake — the most livable urban districts are usually mixed-use, with housing near shops near light commercial space. What zoning prevents isn't mixture; it's unplanned mixture, where an incompatible use arrives without anyone weighing the effect on its neighbours. A site that lets a promotional pop-up interrupt someone mid-checkout has effectively let a tannery open next to a nursery, without a public hearing.
The advertising district problem
Publishers who let ad density creep upward page by page are running an unzoned city: each individual ad placement seemed reasonable in isolation, and the cumulative effect — a page that's more advertisement than content — was never approved by anyone as a whole. Zoning boards exist specifically to evaluate cumulative effect, not just the next single permit application; most content management systems have no equivalent role, which is why "ad density" tends to be everyone's problem and no one's responsibility.
Nobody plans to build a slum, one permitted structure at a time. Nobody plans to build a cluttered page, one approved widget at a time. Both happen anyway.
A site that periodically reviews its own accumulated clutter, the way a planning board reviews a zoning map, tends to stay livable. Most don't, and it shows in exactly the way an unzoned commercial strip shows.