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.

In theory at least...

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.