...Because in the world where people have multiple NFC-enabled cards, this allows one to assign the id of their favoured payment card to their phone - meaning you only have to grab your phone out of your pocket to pay - rather than the current scramble to get your wallet out of your pocket and find/extract the correct card.
Of course however, as someone else pointed out already, this also allows a pickpocket to grab your phone and start paying for things on it too ...