A few reasons why I'd like to pay by phone:

- It's a bit easier to pull out my phone from my pocket vs. pulling out my wallet, then locating my card, and then pulling that out

- Tapping my phone is an easier process than aligning and sliding my card through a narrow slit

- I'd love it if coin-like functionality is one day integrated into my phone, so I can select a specific card to use vs. digging through my wallet for it, or switch cards quickly and try again if the first one gets denied, etc.

These are all admittedly minor, but many successful technologies are about increased convenience anyway.