My opinion only:  
  
Duplicates get closed instead of deleted for several reasons.  

 - seo
 - adding search terms without modifying the meaning of the original question
 - having duplicates closed allows a central way to put all the answers in one place
     - (read as merge)

[Here's a specific example where something got closed and merged to put an answer that merited being seen into the original question][1].  
  
It was originally intended to be an ISO mount/burn question for Ubuntu only.  A later question was asked about burning an Ubuntu ISO for Windows.  
  
It makes more sense to have a single ISO burning question.  It keeps the site clean and centralises canonical answers.  This way we can have one place to point a user with that type of problem.  
  
[1]: https://askubuntu.com/questions/62518/how-burn-or-mount-an-iso-file