Why? from the little info on that landing page, i assume this is amazon's one-click shenanigan migrated to 100+ merchants? or do you go the step further and also handle processing payments? being effectively just a paypal/googlewallet/etc?
For convenience and also being able to re-use that data on mobile. Entering the same data again and again through checkout processes seems and feels like a big waste of time to a lot of customers.