It’s supposed to speed up check out, which it does quite well. The retailer benefits because it makes impulse buys easier and encourages (some) people to check out if all they have to do it press a couple of buttons.
Think of it this way- a first time user goes to buy some coffee, they either enter all their details, set up an account and log in, or they enter 6 numbers…in addition, shop pay has order tracking and history built in if you use the app too.
I actually like it, along with the little ‘code from text messages’ button on iPhones it makes checking out an order almost instantaneous.
Apple/Google Pay also works similarly and makes checking out incredibly quick.
I know people understandably prefer different payment methods. Some people refuse to use anything but cash…
As a side note, I think Shop Pay is handled by Stripe which is one of the biggest e-commerce payment platforms so it’s definitely legitimate. It’s possibly implemented because it’s powered by Shopify which is chosen by a lot of retailers as it’s very simple to use.
If you really dislike it, I think you can opt out of Shop Pay by entering your number on their website which will stop it popping up.