One of the best use cases for this is on mobile. Conversion rates on mobile are really low because users usually don't want to dig out their credit cards when on their smartphone, let alone fill in all their details on a tiny screen. The ability to just tap a couple of times and use their existing billing information (which is synced from their desktop browser) is a huge win. And as mobile becomes the more popular way to browse the web, I think this could be a pretty big deal for online sellers.