1. They emphasize a need.
  2. They hopefully increase the influence on Tableau.
  3. They add new angles and shades not clearly presented in original idea which are helpful for Tableau product developers.
  4. They promote the original idea when someone tells it is a duplicate.
  5. They are sometime much much better written than the original and therefore a better representative of a feature request.
  6. When better written, they attract more up-votes and thus increase the chance of Tableau actually putting it on their product roadmap.
  7. They might only be partial duplicates.
  8. They might not be a duplicate (as for how solution), but only look like a duplicate because they address the same (what is the problem) issues.
  9. They are often made by first time idea contributors. Seeing new contributors who care about improving Tableau's products makes me happy and hopefully many others too.
  10. When addressed issue is a big pain, it might be good therapy to up-vote a long list of duplicates.
  11. They might make other duplicate authors (and of course the original too) feel less alone wanting a particular feature (improvement).