Your login is shared across the website and app.
You can use the app to view content as well as playlists that you have created.
Creating playlists whilst on the desktop site can serve as a useful method to group a collection of courses in one place making it easier to download content when you then switch over to using the app.
App analytics are shared with the website version for both standard 'plays' as well as task viewing analytics, meaning that you can track students' progress whether they view a particular lecture via the desktop site or the app.