Talk about Restartable Active Directory Domain Services in Windows Server 2008/R2. What is this feature good for?

Restart able AD DS is a feature in Windows Server 2008 that you can use to perform routine maintenance tasks on a domain controller, such as applying updates or performing offline defragmentation, without restarting the server.

While AD DS is running, a domain controller running Windows Server 2008 behaves the same way as a domain controller running Microsoft® Windows® 2000 Server or Windows Server 2003.

While AD DS is stopped, you can continue to log on to the domain by using a domain account if other domain controllers are available to service the logon request. You can also log on to the domain with a domain account while the domain controller is started in Directory Services Restore Mode (DSRM) if other domain controllers are available to service the logon request.