Its limiting to dismiss a tool out of hand simply because you haven't encountered a situation where that tool would be useful.

Plenty of web sites allow the end-user to select colors[1], or automatically derive colors from assets provided by the end-user. For those that care about accessibility, they typically calculate contrasting colors to prevent the user from creating a non-accessible experience. A built-in CSS tool like this will, hopefully, encourage more sites to provide a basic amount of accessibility while in no way hindering those who want to build an even better experience.

It would be cool if this was more customizable like the npm contrast-color package but the blog post details why they started with white/black with intentions of changing the algorithm later.

[1] Example: https://coolors.co/8fbfe0-7c77b9-1d8a99-0bc9cd-14fff7