AD is useful for providing policy control over objects (eg for rolling out configurations to a particular set of devices), providing a directory with nested groups for granular access to network resources (file shares, web based systems, desktops, networks, anything that integrates with AD).
AD also provides a lot of logs so you can see wtf happened after an event, or when performing regular auditing.
If you're running a Windows shop you can't deny that AD is your most powerful tool, I've not yet come across anything else that can do what it does. I inherited Samba4 domain controllers and while they worked, switching to AD brought with it increased stability and a full integration with the clients that we could have only hoped for under S4.