One might argue that it had a useful purpose (at least, as long as one sympathises with advert-fatigued users more than with advertisers) in that it means the server providing the advert can't tell whether you're blocking it, and hence can't be tempted to adopt technical measures which (e.g.) refuse to serve you the page content on that basis. All they know is that the advert goes to your browser and a click-through doesn't come back, and they can't tell whether that's because you blocked it before or after it hit your retinas.