As much as I love C# & XAML, I have trouble imagining why I would use this stack instead of writing JS/HTML5 directly.
I could also see a case made that XAML has an arguably much cleaner layout system and styling system than HTML, so some developers may prefer it to HTML.
To develop cross-platform code (e.g. for desktop and web targets)?
Stating that some language is better than another isn't really a good argument, most of it has to do with domain specific knowledge being leveraged to enter another market.