wrong section but good question
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,
2 birds with one stone!