It may not be Azeroth, but it's got a history just as rich and just as convoluted. In the Warcraft universe, the planet Draenor plays a secondary role to Azeroth in terms of storyline -- consider it something like a sister planet, one whose history is irrevocably entwined with Azeroth's. Although these days Draenor exists as a mere shell of what it once was, Draenor, its inhabitants, and its fate are all one of the most significant pieces of Warcraft lore out there. After all, if there were no Draenor, there would be no First or Second wars. There would be no Horde.
Why is this planet so important? It certainly didn't have very much to do with the original inhabitants. In fact, Draenor would have likely lived on in obscurity were it not for the strange, peaceful settlers from another world. Peaceful they may have been, but they had a history they could not escape -- a past that forever linked them with the Burning Legion ... and the Burning Legion knows little of forgiveness or mercy for those that incur its wrath.