What's odd is that they aren't the only guardians on Azeroth. The Aspects were charged to do much the same thing, by the Titans -- each dragon Aspect has its own realm of expertise. Alexstrasza and the red dragonflight watched over all life, Nozdormu and the bronze dragonflight were charged with watching over time, Malygos and the blue dragonflight served as guardians over magic, Ysera and the green dragonflight watched over nature, and Neltharion was charged with watching over the mountains, valleys, and deep spaces of the earth.
All had their own areas of expertise, and we saw what happened to Neltharion -- close proximity to the Old Gods resulted in his going absolutely bonkers. They whispered to him, making him more and more paranoid, obsessed with power, and convinced him that the rest of the Aspects were out to get him cheap wow gold. They talked him into creating the Dragon Soul, an incredibly powerful artifact that pretty much had the power to enslave even an Aspect. They drove him mad, to the point where he was convinced that the Old Gods had the right of it -- that the world needed to end.
When we killed Deathwing at the end of Cataclysm, the Aspects expended the last of their power. They are now mortal, and they have ushered in what they call the Age of Mortals. It's a time in which presumably, we, the mortal races of the world, are now charged with watching over the various aspects of the world just as the Aspects once did. We've proven ourselves powerful enough to sell wow accounts and do so, apparently. Yet here's the curious thing about all of this -- the Aspects and the Ancients seem to be meant for the same purpose. They watch over particular aspects of the world, in order to keep it safe. So ... what's the difference between the two?