Trying to administrate a large number of Windows machines without Active Directory controller support is like trying to ride a bicycle with no wheels, no crank, no sprockets, no chain, and a spike for a seat..
Active Directory supports making RPC calls to Windows. This is used as part of group policies. The RPC calls are used to make edits to the registry and other things on the Windows node.
Using Samba 4 utilities and this feature a administrator on the Samba box should be able to run a virus scanner or other such thing to check the registry settings of any system that is a member of the domain.
Lots of fun stuff like that.
AD is file server, print server, configuration management system, kerberos single sign on, ldap server, and a bunch of other stuff.