Payment Request browser API is a faster more secure version of form payments
developer.mozilla.org
developer.mozilla.org
People stop at the checkout forms because that's often the only place you actually know the price.
There are so many dark patterns adding fees, adding shipping cost, or signup to newsletters, that the only place you know what you will be charged is when you're making the payment.
It's not unlikely that people click through to checkout, just to find out what a thing will cost.
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On topic, there certainly are place online where I'd rather use a browser based payment flow that ideally doesn't involve me sharing credit card numbers with a merchant.
I guess this helps to provide a similar experience online, but with support for all payment methods.