Game portals place their own adds on the same page as the game but not inside the game unless you import and use their API. A few game portals have profit sharing on these same page adds but most do not.
If I were able to display a fully functional web page inside the game I could use my own ads or use the displayed web page to display third party adds in the game.
In addition I could also give away prizes (such as free downloads) for winning the game or getting a certain score. The displayed web page would either contain these prizes or links to these prizes.