It's lore and why people care about Warcraft lore I have no idea. It's all the same formula: Dark thing has returned named X, you must do Y to take care of X. Etc etc.
And then you do it and nothing changes in the game. They also use the lore to justify stupid gameplay decisions. Example #1!
There is a Druid spell called Entangling Roots, it is the crowd control for the class. Crowd Control is one of the biggest aspects of WoW. Unfortunately, the spell can only be used outdoors which makes it useless in dungeons and many other places. Why not change this? Because the lore says Druids have more power outdoors, so they only get outdoor ability. Yet the mobs use the same spell indoors all the time. The response to this? A mighty shrug and whimpering about lore.
The story is lame and how it's used is even lamer.
There is a few interesting threads of story, but sometimes they just never finish them. An alliance quest involving a burned down building and a crazy hobo is actually a bit intriguing, but then the quest line suddenly stops because they didn't bother ever finishing it.