Large sites have complete marketing departments to sell their website. Here i am talking about the marketing of the site itself and not its product. For example, no doubt Yahoo is great, it is doing nice, but yes it needs to rope in new consumers to increase its hit rate and its number of registered users. how does it do that? Marketing. yahoo has complete bilboards in major cities, has radio jingles, Tv ads, etc.. all cajoling the consumer to get online and use Yahoo.
Not all sites can afford that. So what do they do? Advertise on the web itself, pulling people towards it. This is like a TV channel placing its ads on its own channel. how can such a concept work? it will, on the web medium. it does atleast as far as building a recall is concerned.
The ad above is of such a site. It uses clever funny flash animation which sticks in the minds of consumers for long.
One obvious advantage with such a media is – LOW COST, or rather no cost, except for opportunity cost probably.