It provides a suite of XAML related analysis, code generations, and utilities to help in Xamarin.Forms development - especially MVVM development.
This is where a new productivity tool, MFractor, aims to step in to the void left by Visual Studio Mac. Even when XAMLC is on, you still have to compile before finding any errors, and forget about any refactorings.
Sometimes intellisense is there, sometimes it isn’t. One of the pain points though is that Visual Studio Mac (or even VS Windows) doesn’t treat Xamarin.Forms XAML as a first class citizen. One, I get to use C# (as opposed to Swift or Java), if I'm lucky I'm able to develop a complete app with almost 100% shared code, and the Xamarin.Forms framework is pretty well put together - it sports a great binding engine, I find the abstractions over the common controls covers at least 85% of the day-to-day scenarios I run into, and I can dive into the platform specific code if needed to really customize my apps. I find Xamarin.Forms development rather enjoyable.