The first reason why Adsense is vital for content sites is because it has come a long way in understanding the needs of publishers and webmasters. Together with its non-stop development is the appearance of more highly developed system that allows full ad customization. 

The second reason is the capability of the Adsense publishers to track not only how their sites are progressing but also the income based on the webmaster-defined channels. The new improvements in the search engines gives webmasters the ability to monitor how their ads are performing using customizable reports that has the ability to detail page impressions, clicks and click-through rates. With real-time reporting at hand. 

The final reason is that the advertisers have realized the benefits related to having their ads served on targeted websites. As a result, increasing the possibility that a web surfer will have an interest in their product and services.

There was a time when people were not yet aware of the money to be achieved from advertisements. The cash generated only came into existence when the webmasters and publishers realized how they can make Adsense be that producer. In those days, the content was the most important factor that was taken quite seriously. It still is. With the attraction of money.