DAC feature added from the SQL Server 2005 version. Using DAC feature a Database Administrator can connect to a SQL Server Instance when the database engine is not responding to regular connections and troubleshoot the issue.

Advantage of using DAC is DBA can connect to a SQL Server Instance and execute T-SQL commands to troubleshoot and fix issues rather than rebooting the SQL Server which could lead to database corruption or other problems.

By default DAC is disable, it is a best practice to enable the DAC.
We can enable the DAC using below T-SQL command.

Use master
GO
sp_configure 'show advanced options' , 1
GO
sp_configure 'remote admin connections', 1
GO
RECONFIGURE
GO

In SQL Server 2008 to enable the DAC, right click on SQL Server and Select facets and from facets drop down Select Surface Area Configuration.