What's the main reason why the Department of Mysteries keep prophecies?
The Keeper of the Hall is a Bureaucratic witch or wizard who orders and maintains the Records placed on the numerous shelves that comprise most of the Hall's interior. Presumably, after the Records are correctly assorted, stringent anti-theft spells are placed upon each and every one of them, allowing only those to whom the Prophecies refer the authority to remove them from their places. http://harrypotter.wikia.com/wiki/Prophecy_Record
As far as I know, the Department of Mysteries is the department where they put recordings of prophecies, and that includes the prophecy about Voldemort and the one who's supposed to conquer his power. They are recorded and then put into those glass orbs.
Well, for this part, it seems reasonable that this is one of the most important prophecies since the Ministry of Magic wanted to find out how to conquer Voldemort's lust for power.
But I'm puzzled with this. Why do they even keep them? What significance do those prophecies have? There may also be certain items there that are very important or can play a great role for magic. But I can't remember why they keep these prophecies. Do they need them for future reference, fate guesswork, etc.?
Things to remember:
The Department of Mysteries could have just stored the important prophecies. So there's a possibility that they stored those prophecies for an important reason or maybe a reference.
I have a theory that they stored those prophecies for future references. There's a possibility that they will need it for a certain reason which I don't know.