Previously Microsoft created a visual framework which they used for web and Windows apps internally which then they expanded and are now sharing with the global audience. The most noteworthy use of FD is apparently Microsoft Office's apps for mobile.
The problem (except the framework's name Fabric, I thought they bought Twitter's and then Google's crash reporting system, ugh) is that this is just a UI framework, like the unofficial material design (not by Google who created Material design) UI framework for iOS.
FD is NOT a cross-platform runtime framework like React Native (written with ReactJS) or Flutter (written in Dart), but it's just a UI framework for iOS (written in Swift) and Android (Kotlin) which you can use in your apps.
So if you're a fan of Microsoft's buttons or their pickers you can go for it, but since most of current apps are using either stock iOS or Android UI elements or some own custom designs instead, I can't see much use of FD just yet.
Also currently their github repo has only a single issue with a question about FD's roadmap which is now unanswered. Maybe the answer will provide any notable information on the future of the framework but at least for now it's really hard to evaluate its worth, especially at this early stage.