he can surpress too and uchiha are better to be NINE TAIL jin. not all tails but only NINE because they can control and suppress its chakra, and i think he would do it so sasuke would be able to help naruto, think about it naruto was only fast AFTER he went TB mode, and the only way to get close to obito is to use flying thunder god in some way, or jus being really fast so, who's to say he want do it to help give sasuke speed to help fight obito with his son before obtio kill him and the 2nd, i mean dude only has one hand going heads up with the 10 TAILS, id think he is going to last that long