Any iPhone, iPad, or iPod touch with at least 32GB of storage comes with Keynote, Pages, Numbers, GarageBand, and iMovie preinstalled.* Users with an iTunes account can adopt these apps the first time they open them. This gives them a license, and adds the apps to their purchase history.
If you use MDM to assign and distribute content to users and devices, you can use MDM to manage these apps and assign them to devices. This way, users don't have to sign in to the App Store to adopt them.
* If you purchased a device with iOS 9 or later that's 16GB or smaller, or if you updated your existing device to iOS 9 or later, you can download the apps for free.