Paying by smartphone is a solution in search of a problem.
You'd think that (hell, I'd have agreed), but people already pay by smart phone even without Google Wallet or Apple Pay. The obvious example is people paying by the Starbucks app, although there they get free drinks for paying that way so whatever. (However they could also pay by card, the app is just another method of getting free drinks.)
Another example is that you can apparently pay by smartphone in the cafeteria at the company I work at. I have no idea why you'd want to do this, but it's another "scan the barcode" thing, and people do it.
I have no idea why, but apparently some people find pulling out their phone and using that to be easier than using a card. Even when it takes longer because I'm not talking about NFC systems here, I'm talking about scanning a barcode on a phone.