One advantage might be the ability to support multiple platforms. Using the technology group formerly knows as the "web stack" you could build a version of your app for windows desktop, ubuntu desktop, browser apps, mobile, and probably others.
Other than there being a lot of people who already know JavaScript but don't know C#.
I think this argument could have been made with Nodejs and React Native, but we've seen how those worked out.
"when you've got a much better language in C#"
That's subjective.
C# can accurately subtract decimal numbers without having to invoke a webservice or use a third-party library. I'd call that better.
(Note: I spend most of my time writing JS, but am extremely familiar with C# and Java)
It's not subjective. JS is only relevant due to ubiquity. Nobody transpiles to C#. (well, except for Haxe - to nitpick, but it also transpiles to Java and C++).
Because people compile directly to CIR. See: VB.net, F#, Scala.net, IronPython, IronRuby, etc.
I meant C# is awesome so you just use it directly :).
Gives you options. If you have an existing web app, you can likely reuse the bulk of it to quickly get a Windows app.