During the time of the Oblivion Crisis, it was said that Skyrim was hit especially hard by the Daedric invasion. Yet, a mere 200 years later there isn't a single sign of this event ever having happened anywhere. Such a monumental event in history would surely have left its mark, especially in the cities. When Martin Septim made the ultimate sacrifice to seal the barriers to Oblivion forever, this did not remove the structural components that would be left behind by suddenly having the dimensional gateway closed down without warning. While logic may suggest that the citizens of Skyrim would have done all they could to be rid of these menaces, I don't believe this would have been a top priority. Some might even have left them standing as a reminder of what almost was.
This mod's purpose is simple and to the point. Each of the 5 major hold capitols has an Oblivion Gate either inside or near it. They are merely for decoration and will not be made functional in any way. They're simply there to add back the missing atmosphere that Bethesda left out. An homage to Oblivion (the game) if you will.