They bring a whole new option to cross platform development: an abstraction layer that lets you write once, and run native on each of the platforms.
Write Once, Render Natively
This means, for example, when you create a tab page, it is rendered as a native tab page on iOS, Android and Windows Phone. This is accomplished using pre-defined elements each of which has a hidden renderer for each platform. Most of the time you don’t know or care how it is done, unless you want to create a custom (or customized) control.