Blogger started to display a block of contextual ads after publishing blog posts. Until now, Google's blogging platform didn't include ads and it was only monetized by making it easy for publishers to use AdSense ads.
It seems that Blogger followed the advice of Evan Williams, the co-founder of Pyra Labs, Blogger's parent company: "Put Ads in the App Interface: For example, the published landing page alone must get millions of views a day. And it's the perfect point for someone to go elsewhere. See if you can target it off what they wrote about. No one will mind (much). And strengthening your revenue story will strengthen your position in the company."