It has massive perks, especially in regions where the transaction fee or exchange fee scams run.

(Basically they have a reasonable price showing and when you pay with a credit card you see your banks charges you a huge amount. It can be as simple as in Mexico $ is used for pesos so if the merchant actually charges you USD that's 20x or so rip off).

But... This isn't mutually exclusive, why can't I set a price on my smartphone when I pay by contactless?

The other benefit is that there QR codes are bank transfers. Which have substantially less recourse and less rights for the consumer. Probably doesn't matter for a $10 sandwich but still significant.