hmm, maybe he wanted to attack konoha with help of kyuubi, and because only uchiha have ability to control the kyuubi, the higher ups like hokage,danzo and other old farts will think uchiha did it, this will result in a conflict between uchiha and the leaf.
then some old fart would ask some good ninja to kill all uchiha, obito just want to take the advantage of this and remove all uchiha from his way, and get all those extra sharingan eyes,
Because madara hated kohoha maybe or to get the 4th to use the death ripper seal jutsu so he could die and not worry about him or because he wanted kohoha to feel the pain that he felt when rin died idk could be anything