Further adoption of just using your phone in replacement of your whole wallet.

Right now when I need to carry my ID I still just end up carrying my wallet. The wallet full of cards not tied to Google Wallet. When I buy things with the Google Wallet tap to pay I imagine they're getting some amount of metadata about that transaction.

Also, its yet another feature for Google to have over Apple, or vice-versa. If one side gets it, the other will probably want to match the feature.