There are two reasons one might want to use Google Pay / Apple Pay contactless payments: (1) The app does not share your actual credit card number with the merchant, instead a virtual card number is exchanged with the POS terminal. So it's a bit safer for you in that your credit card number cannot be stolen and misused somewhere else; and (2) of course you don't have to carry the cards with you and risk losing them somewhere and being used fraudulently. Or to flip it around, if you do lose your wallet but still have your phone, it's not the end of the world, and you can still use your cards loaded in the app for payment.