I was going to start a business in this space, and I can sum it up as broadly this:

- It saves all the details needed for 1-click, not only your card (which is just PayPal). So: shipping address, name, possibly even 'pre-done' age or fraud checks, etc.

- It 'maps to' the business's API far better than a browser's autofill can do, which only has some random HTML tags and a helluva lot of guessing. (In other words, users won't have to keep filling in the, like, 4 out of 9 inputs that it didn't understand.) Your advantage is that the merchant itself does the integration work themselves - though ideally your API/SDK will make that as easy as possible.

(Essentially, it's just: "user has a third-party cookie for your site, comparable to SSO, and you charge their saved details and call the merchant's API on the backend [or, for small businesses, a Shopify integration or whatever fresh hell the Fast engineers must've gone through]".)

It's an enormous potential market, though that's also true of the shipping industry and it doesn't guarantee big margins. I'm interested to see what comes of it all. I expect something boring like "Yeah, eventually [company X] got big enough, that's why Apple released the Apple Pay Checkout Fields API for JS".

As a footnote: Amazon has actually offered this as a service for ages, which I know because of the precisely one website I used which happened to offer it. If it weren't for the terrible antediluvian UX and the lack of any evident marketing, it probably wouldn't have been such an open goal for these startups. Here, this is what Fast was trying to sell a shittier version of, without a user base: https://pay.amazon.com/what-is-amazon-pay