Cross-platform capability is not the only possible motivation to use something like this. Personally, it interests me because I don't see WinForms/WPF/UWP as viable, so right now the web stack is the only capable high-level UI toolkit on Windows. On MacOS, you can still get by with native.

For an app I'm working on, I'm exploring the possibility of a shared core in C#, with the UI layer rendered with the web stack on Windows, and with Xamarin.Mac (native binding for .NET) on MacOS.