To know why Ad-sense is essential for your content sites is to know first how this works.
The concept is really simple, if you think about it. The publisher or the webmaster inserts a java script into a definite website. Each time the page is accessed, the java script will pull advertisements from the Ad-sense program. The ads that are targeted should thus be related to the content that is contained on the web page serving the ad. If a visitor clicks on an advertisement, the webmaster serving that ad earns a fraction of the money that the advertiser is paying the search engine for the click.
The search engine is the one handling all tracking and payments, providing an easy way for webmasters to display content-sensitive and targeted ads without having the bother to solicit advertisers, collect funds, monitor the clicks and statistics which could be a time-consuming task in it. It seems that there is no lack of advertisers in the program from which the search engine pulls the Ad-sense ads. Also, webmasters are less concerned by the shortage of information search engines are supplying and are more focused in making cash from these search engines.