The goal of Math SE is to create a searchable repository of high quality questions and answers.  "Searchability" is important—if the same question is asked over and over again, then it becomes difficult for search engines (or users) to find the *best* answer to a given question.  Duplicates create a lot of noise and clutter which make navigating the site harder.  Closing a question as a duplicate helps to organize the site, and helps search engines to find canonical answers.  Thus the point of closing an old question is to improve site organization and accessibility.

Regarding the specifics of your question, while the questions themselves are not identical, answers at the dupe-target seem to address your question.  The point is not that the *questions* are duplicates, but that the question *marked* as a duplicate is answered at the dupe-target.  Questions need not be identical in order for one to be closed as a duplicate of another.