While speaking at D8, Steve Jobs addresses this directly, speaking about why Apple went into advertising, and what it means to the company.
“We’re going into the ad business because we want to help our developers make some money.”
Noting that people do considerably more application work on devices than they do on desktops, he then states that “If you want developers to make money, you put ads in the apps”.
Of course, the thought of Apple making $2bn doesn’t hurt the reasoning, either.
As for the advertising experience? Jobs relates that presently ads will take you out of an application and into a browser. “If you are playing a game, you are probably not going to make it back to the same place”.
When asked if someone else could do it: “Sure, but nobody else is doing it. We can build it into the OS.”