A user applies their judgement to cast a duplicate-question close vote. As far as I know, other than the automatic comment posted under the close-voter's account, until the post closes, there is no other indication of which user first applied their judgement to cast that first duplicate-question close vote. If I disagree with that judgement, the auto-comment left by them provides me a mechanism to @notify them in a comment explaining why I disagree- especially in the case that I don't think the question needs further clarifying (yes, there are cases when this happens). Being able to ping that user means they get a chance to see a different application of judgement, which may lead them to retract their close vote.
Canned comments in many review queues are left under the name/account of the reviewer, providing the same benefit. Editors are pingable in comments. See also How do comment @replies work?.
Also, having a duplicate-question comment posted under myself when I cast the first duplicate-question close vote allows me to edit the comment.