Board Thread:Theories and Speculation/@comment-45444155-20200507075527/@comment-6769189-20200513205656

Attacking Obito with the Senjutsu Rasengan had 2 purposes
 * to injure Obito
 * to test if Senjutsu would actually work against the TSB

They knew that the TSB was fast enough to get inbetween Obito and their attack so if their theory was correct they could land a surprise hit on Obito