That's how much money has been generated for developers through Apple's App Store. When people are looking around at what platform to code for, there are many reasons to choose any of the platforms, but the main consideration is money. And with today's announcement of a solid revenue stream, everyone will want some of the (ahem) Apple pie.
Speaking of pie, Apple's 30% cut means it earned $3.5 billion on all those Applications. How many phones would they need to sell to clear that as a margin?
I digress. It's impossible to argue that the smartphone world is not app-driven, and that means developers will be courted by all the manufacturers and OS vendors to ensure the code will be for their platform first. But the greatest tool is a mix of word of mouth, and a genuine belief you can strike it big with your next app.
Even if the numbers say it can be done, there's a belief that Windows Phone is too small, BlackBerry 10 is unproven, and there's an issue with piracy on the Android platform (and a lower average spend on applications compared to iOS as well). Today's $8 billion wasn't just about getting the number out there, it was to continue the temptation of the developer - and to challenge the other platforms to show developers they could be just as successful on a platform that isn't iOS.