What is "AADJ" ?
Azure Active Directory Join is the functionality that registers a device in Azure Active Directory to enable centralized management of the device( As AD On Prem use to). It makes it possible for users to connect to the cloud through Azure Active Directory.
Azure AD Join is intended for Small to Large Companies that do not have an on-premises Windows Server Active Directory infrastructure.
Reducing On-Prem Foot Print?
If you are focusing on reducing your on-prem footprint and wish to focus more incloud, Azure AD Join could benefit you. You can create Azure AD accounts manually or you can let it sync from your onprem AD. After the Computers are added to the "Domain" single sign-on could be set up as one of the few Sign On features provided by Win10.
Devices can be automatically enrolled in mobile device management when connected to Azure AD. This process works with Microsoft Intune and partner mobile device management solutions. When device management is done with Intune, IT administrators can monitor/manage Azure AD-joined devices.
Single sign-on from the Windows to apps and resources in the cloud, such as Office 365 and thousands of business applications that rely on Azure AD for authentication through Azure AD Connect.