My guess is that originally obito wanted to get back his other eye from kakashi but he had the problem with the gedo mazo. He needed someone to control the gedo mazou and so he was gona sup up sasuke in order for him to control the mazo. This way obito could get his other sharingan back and be so broken that he is unbeatable unless you are edo madara.
well he recruited hawk (or whatever sasuke called his dumb little group) to help capture the 8 tails when the akatsuki was running low on members (he didnt do a very good job anyway)
as far as his current roll in the story... I think he's out of the war arc and he's going with oro to set up the story after the war is over. No one said the story is going to end after obito and madara. we could very well see a part 3 in which sasuke is the final antagonist. I think kishi is setting up something bigger than we can realize right now.
As others have said Obito was just using Sasuke. He used Sasuke and Taka to hunt down the eight tails due to the Akatsuki dwindling numbers. He had no real role in Obitio's main plans but he could be easily manipulated and used to help out.