1/ I'm thinking of Apple pay now. The convenience lies in the fact that you don't have to type in your PIN / sign the transaction receipt. Also, when paying with cash you get change. Personally, I hate change. But I don't think I'll be leaving my wallet at home any time soon.

2/ Not all taxis (at least in Europe) have the possibility of paying by card. Using an app gets rid of that problem. Also, if you're using taxis in a business context, you get automatic receipts.