The entire web is an advertisement.

Think of the web as a magazine, or TV, or life. Websites are the content, like articles, or TV shows. Corporate websites, promotion websites, are advertising - like magazine ads or TV commercials. E-commerce and banking websites are the transaction centers of the web.

Banner ads are one form of advertising. Focusing on them in a "web advertising" article is no different than assuming the only form of "real-world" advertising are billboards. Banner ads use power words like "FREE" due to limited screen real-estate. Those words are very powerful and are proven to receive significant attention.

Websites, on the other hand, are like a 2-page spread in a magazine. All the room to sell with compelling copy is available. Banners are like a billboard. It has to sell in 3 seconds. The quickest way to do that is to grab attention. While you don't want to hit people over the head to do it, you have to find a way. Otherwise, your banner fails.