Last time I paid for a round in the pub by tapping my phone against the card reader, someone asked me if there was a purpose to such things, other than showing off.
There is (though I personally like the showing off in any case). When you pay with your phone, instead of using the same fixed 16 digit number printed on the corresponding credit or debit card, it uses a different number for every transaction (with a handful of exceptions … for example, it’s smart enough to keep it the same when touching in and out on the London Underground). This means that anyone getting hold of your card number from the merchant receipt printed out by the till can’t use it for another transaction, unlike when you use the actual card. OK, so they’d potentially need to guess other details in either case, but it does provide a decent level of protection over and above paying with a card.
It’s also worth knowing that the normal contactless payment limit of £30 doesn’t apply to cases where you pay with your phone (because the phone can challenge you for a fingerprint or other secondary authentication). Provided the till supports it (which is rare, but getting more common – try a petrol station), you can pay up to £100 (but your phone will ask for a PIN or fingerprint when it’s above £30).