Oh yeah, there are definitely valid use cases. If you’re a web hoster, you host many sites under user’s domains. There are plenty of features you might want to offer your users — for example, if they’re logged into their hosting account, visiting their domain shows them some kind of status or site editing bar. Or maybe there are social features, like liking posts or commenting on other people’s sites without having to create new accounts and logging in everywhere. 3rd party cookies make this possible. Alternatives (at least as of a couple years ago) are often still worse user experiences.
For a modern example, restaurants have online ordering systems. And a lot of them use the same service under the hood. (Eg toast.) If you want to use the same credit card you used somewhere else, you have to login on every single restaurant site using the SMS code. (Eg “link pay.”) Allowing 3rd party cookies would make that flow faster, since you could visit other restaurant’s domains while still being logged into the 3rd party payment domain. (And specifically, logged-in inside an iframe so the restaurant site can’t read your payment info.)
These flows all feel very dangerous to me because they potentially allow a site to access information about me that I have not explicitly allowed.
Take the web hosting example; naively if I visit any site hosted by that company, can they detect that I have an account and am logged in to my hosting account? That feels like a dangerous amount of leakage, and you're relying on the hosting website to make the correct restrictions rather than having it structurally embedded in the user agent.
The shared payment system feels even worse -- is it then possible for a random website to get a payment through this system, or extract information about my payment account?
For a modern example, restaurants have online ordering systems. And a lot of them use the same service under the hood. (Eg toast.) If you want to use the same credit card you used somewhere else, you have to login on every single restaurant site using the SMS code. (Eg “link pay.”) Allowing 3rd party cookies would make that flow faster, since you could visit other restaurant’s domains while still being logged into the 3rd party payment domain. (And specifically, logged-in inside an iframe so the restaurant site can’t read your payment info.)