DAC attracted my attention at the moment when I was unable to connect to the SQL Server using a traditional way (the reason was deleting logon trigger). I made some searches on the web and it turned out that the only way to access the Server was to use DAC connection which unfortunately was turned off by default. However, I was able to overcome the problem (I removed trigger) using just sqlcmd.

My question is if I can connect to the SQL Server just using sqlcmd, what is the purpose of DAC then?

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    DAC also reserves cpu and memory for you so if your server is so hammered that it can't respond properly to other client requests, it always has something available for DAC. see brentozar.com/archive/2011/08/… – Jonathan Fite Dec 2 '19 at 13:37

Referencing Diagnostic Connection for Database Administrators with highlighting mine:

SQL Server provides a special diagnostic connection for administrators when standard connections to the server are not possible. This diagnostic connection allows an administrator to access SQL Server to execute diagnostic queries and troubleshoot problems even when SQL Server is not responding to standard connection requests.

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