Dark Void is Darkrai's Signature move, and cannot be mimicked by any other Pokemon, but it can learn Hypnosis.
Dark void only has 50% accuracy while Hypnosis has 60%.
And almost no body wastes a move slot on Mimic.
So Is Dark Void just useless?
The only advantage in using Dark Void over Hypnosis would be in double or triple battles, since Dark Void targets all adjacent opponents and can put multiple Pokemon to sleep at once as a result. Also, it had 80% accuracy prior to gen 7 so it was a much better move then, even in singles.