Apparently certain workflows in Okta would be hampered or prevented if third cookies were totally blocked.

https://support.okta.com/help/s/article/FAQ-How-Blocking-Thi...

Once case is iframing a site where people will get logged-in behavior without requiring them to log in to each site individually. For example, an embedded video player that wants to disable ads for subscribers or a micropayments service that needs to recognize which account to charge.

(Disclosure: I work on ads at Google, speaking only for myself)

They're used for fraud prevention. How you're tracked is similar to ads, but the end goal isn't targeting ads better, it's preventing chargebacks.

A lot of older SSO technology like Gigya use 3rd Party Cookies. They've had a hell of a time on Apple devices.

I had to enable third party cookies to log into Pearson’s online learning garbage. It appears the use case here is standing up poorly designed websites.

Basically it’s deprecated technology, anybody using for “legit” reasons needs to migrate to a new solution anyway.