Funny things happen when you use your phone with a transit system. What happens when the battery dies after boarding, but before fare inspection? What if you need to tap out, but can't? As phones can be used as credit/debit cards, and some transit systems allow you to tap in and out of them with phones, this is a valid concern. An app that allowed you to reload your card using your phone would e a solution, but that's a bit complex, and there is huge range of phones that do and don't support NFC.