Cause he's a retard. I don't think he needs one now but only since about the time he unlocked BCM because he can use other types like mini rasenshurikan which is still a lot more advanced than a normal rasengan
Cause he's a retard. I don't think he needs one now but only since about the time he unlocked BCM because he can use other types like mini rasenshurikan which is still a lot more advanced than a normal rasengan
he was not able to concentrate enough to maintain the shape of rasengan and release the chakra at the same time.
so he releases the chakra while his clone maintain its shape and rotation,
and in case of rasenshuriken there is another clone for nature manipulation.