This can be asked in many ways, but why is there a specific dell-r710 while there aren't for r200, r610, r620... etc etc?
Do we really want a tag for every model?
Isn't poweredge enough?
Just a dell-poweredge tag seems plenty to me.
The simple answer is: because everyone with >= 300 reputation can create tags by just specifying them with her question:
Simply enter a new tag with your question and it will be created.
Somebody chose to do so and others tagged their questions with the same tag afterwards.
Tag cleanups are performed on an irregular basis by the SF regulars and moderators by means of manually re-tagging all questions using a "bad" tag and deleting it afterwards. If you want the dell-r710 to disappear, just post a new question making your case.