One thing to keep in mind is there is no one official arr stack. It is dozens of mix and match services, which leads to the confusion. The very basics are the ability to automatically download movies, TV shows, music, and ebooks, as others have covered. On top of that, there are services that support the core download services. Like to provide VPN to those downloaders, automatic transcoding to other formats, automatic renaming of files and copying to specific folders, or web interface to request new content. And that is a small sample of what you can do.