Why is the iPhone 5 so important financially?

Because the iPhone appears to be responsible for the vast majority of Apple's profits.

A couple of months ago, in a lawsuit with Samsung, Apple was forced to release documents that showed the relative gross profit margin for its iPhones and iPads.
This information allows us to estimate just how much profit Apple derives from the iPhone.

The bottom line?

The iPhone likely accounts for almost two-thirds of Apple's profits.