> Isn’t this the same as “pay with PayPal/Amazon pay/etc”?

There is always the issue of how well the integration performs. It is not uncommon for merchants software to get confused in the process and ship your order to the billing address or for other things to go wrong. When that happens, you now have to deal with customer service of varying quality.

In most cases, the purchase decision is easier for me to make on Amazon than anywhere else. I've been a customer since almost the first days and while I can count a lot of minor irritations, there isn't anything better.