Pay per website you visit, you visit a lot of different websites.
The notion is that you make each payment to be the smallest atomic component, so you need high-volume for that (since one large purchase gets split into many small purchases).
End-users don't like being scammed. If it's sufficiently low value with sufficiently high service level, you will get adoption. Appstore payments of $0.99 don't even register for a lot of people (with the exception of egregious Pay-to-Win games), if the payment was $0.001 and you only end up spending $0.10 in a day nobody would care. If you're getting billed per-hour on AWS, people prefer that to per-month.
The benefit between companies isn't for persistent relationships. It's the notion that you're reducing information cost (instead of paying a large payment with signon, you don't care who your counteparty is -- if they don't deliver who cares you're only out $0.001). In the longer-term, this is about reframing how you discover business relationships in the first place due to information/transaction costs.
Marketing.
I'm also suspicious of pay-per-whatever extending to end-users. People hate that kind of billing, even when it's cheaper (I learned this the hard way). And between companies, again, what's the benefit of having thousands of transactions when a couple would do?