I'm no mobile developer but I guess that being able to share classes and libraries(that are non UI/mobile specific, mostly services and domain/business logic) among all the .NET platforms. So you could use the same libraries for an android,ios, windows phone client, a desktop GUI client, a webserver, a static website.
You have access to all of the same namespaces that a native app does, and also access to all of the namespaces included in .net Framework Standard.
You can stay within .net Framework Standard and share that code between your Android/iOS/Windows Phone(lol) app, and reach into the native libraries when you need platform specific functionality.
if the xamarin cross platform sdk maps closely to one mobile platform, how does make it intuitive for iOS developers? Or are the classes aliased for each target platform?
In theory at least...