Earlier quoted context omitted.
By driving more eyeballs to the web and away from desktop apps.
Really ? So you'll be working away, on your document or your spreadsheet, or you'll be playing a game, and some ad will pop up, and off you go, zero attention span? Why would anyone want that? $50 on your Netbook isn't much if it means you hand your thesis in on time...
Selling advertisings isn't about creating new places to put ads--it's about making advertising space more valuable. And that isn't as straightforward as you seem to think. :)
* And adsense ads in apps that others create.