To know why Adsense is essential for your content sites is to know how it works. The concept is really simple. The publisher or the webmaster just need to inserts a java script into a certain website. The java script will pull advertisements from the Adsense program each time the page is accessed. The ads that are targeted should therefore be related to the content of the web page serving the ads. If a website visitor click on an advertisement, the publisher serving the ad earns a portion of the money that the advertiser is paying the search engine for the click.
The search engine is the one handling all the tracking and payments. Google , Yahoo or other search engine providing an easy way for webmasters to display content-sensitive and targeted ads without having the hassle to solicit advertisers, collect funds, monitor the clicks and statistics which could be a time-consuming task in itself. It seems that there is never a shortage of advertisers in the program from which the search engine pulls the ads.