Could someone explain why Adblock injects CSS rules into the document? I presume it's to hide the ads, but what I don't understand is why that's the way it hides the ads. Why not simply remove the element containing the ad¹ instead? Additionally, presuming my earlier assumption is correct, if you hide the element with CSS, doesn't the browser still need to fetch the ad?

¹Get rid of as much crap ad HTML as possible. Ads seem to come layered in all manner of divs and iframes.