Not in terms of UI, no. If you write your non-UI code in shared libraries that are PCL or .NET Standard and have your web-server use them too, Xamarin enables

Not in terms of UI, no. If you write your non-UI code in shared libraries that are PCL or .NET Standard and have your web-server use them too, Xamarin enables that scenario, but doesn't really bring anything else to the table for a web app.