In the early days of personal computers, killer apps were a luck-driven factor in the success of platforms. VisiCalc in the case of the Apple II, and Wordperfect in the case of DOS.
Observing how killer apps are crucial, the makers of risky, new platforms have since attempted to load the dice in their favor. This is particularly the case with game consoles.
Just look at how Nintendo and Sony line up games for their consoles, these literally make or break it for them.
It wasn't an acquisition, but Office was the preordained killer app in the early Mac days. Apple pulled MS in with unparalleled access to their new machine in order to get Word/Excel out in time.
Later, as we all know, Windows became the main platform for Office apps, and Office on the Mac was seriously neglected. So what was one of the first things Steve Jobs did upon returning to Apple? He persuaded his old friend Bill Gates to promise to get an updated version of Office on the Mac. This turned what seemed to be sand into concrete.
I'm forgetting what the original killer apps were for the iPhone, but I'd guess that, like MS with Office, Apple focused on their own music, photo, and chat apps.
